How do CSS triangles work?

T

TylerH

CSS Triangles: A Tragedy in Five Acts

As alex said, borders of equal width butt up against each other at 45 degree angles:

https://i.stack.imgur.com/hZefy.png

When you have no top border, it looks like this:

https://i.stack.imgur.com/uV9Q5.png

Then you give it a width of 0...

https://i.stack.imgur.com/K1A7G.png

...and a height of 0...

https://i.stack.imgur.com/NsmsW.png

...and finally, you make the two side borders transparent:

https://i.stack.imgur.com/B42zY.png

That results in a triangle.

@Jauzsika, you can add these triangles into a page without adding additional elements simply by using :before or :after pseudo classes.

Act with animations: jsfiddle.net/pimvdb/mA4Cu/104. Just for those of us who need even more visual proof like me...

With different left-border and right-border, non-isosceles triangle can be made. And when many triangles are combined... jsfiddle.net/zRNgz

In 2018, is there a better way to make a triangle with CSS rather than using this hack?

@Scribblemacher Inline SVG would do it.

j

johannchopin

The borders use an angled edge where they intersect (45° angle with equal width borders, but changing the border widths can skew the angle).

https://i.stack.imgur.com/nqXnr.png

div { width: 60px; border-width: 30px; border-color: red blue green yellow; border-style: solid; }

Have a look to the jsFiddle.

By hiding certain borders, you can get the triangle effect (as you can see above by making the different portions different colours). transparent is often used as an edge colour to achieve the triangle shape.

Y

Yassin Hajaj

Start with a basic square and borders. Each border will be given a different color so we can tell them apart:

.triangle { border-color: yellow blue red green; border-style: solid; border-width: 200px 200px 200px 200px; height: 0px; width: 0px; }

which gives you this:

https://i.stack.imgur.com/QH01k.png

But there's no need for the top border, so set its width to 0px. Now our border-bottom of 200px will make our triangle 200px tall.

.triangle { border-color: yellow blue red green; border-style: solid; border-width: 0px 200px 200px 200px; height: 0px; width: 0px; }

and we will get this:

https://i.stack.imgur.com/3gohz.png

Then to hide the two side triangles, set the border-color to transparent. Since the top-border has been effectively deleted, we can set the border-top-color to transparent as well.

.triangle { border-color: transparent transparent red transparent; border-style: solid; border-width: 0px 200px 200px 200px; height: 0px; width: 0px; }

finally we get this:

https://i.stack.imgur.com/k9EQ8.png

Cool, but isn't this the same way? :-)

There's another way to draw .., which turns out to be the same way :) Nice explanation though

-1 for using JPEGs with massive artifacts. But +1 for creating a great example of when not to use JPEGs that I can link to in the future for deterrance. ;)

why isn't a gif used instead here?

Sorry @hheimbuerger, I messed up your example by fixing the images. You’ll have to link to revision 2 of this answer in the future.

m

maxshuty

Different approach: CSS3 triangles with transform rotate

Triangular shape is pretty easy to make using this technique. For people who prefer to see an animation explaining how this technique works here it is :

https://i.stack.imgur.com/63AGJ.gif

Link to the ANIMATION : How to make a CSS3 triangle.

And DEMO : CSS3 triangles made with transform rotate.

Otherwise, here is detailed explanation in 4 acts (this is not a tragedy) of how to make an isosceles right-angled triangle with one element.

Note 1 : for non isosceles triangles and fancy stuff, you can see step 4.

Note 2 : in the following snippets, the vendor prefixes aren't included. they are included in the codepen demos.

Note 3 : the HTML for the following explanation is always :

STEP 1 : Make a div

Easy, just make sure that width = 1.41 x height. You may use any techinque (see here) including the use of percentages and padding-bottom to maintain the aspect ratio and make a responsive triangle. In the following image, the div has a golden yellow border.

In that div, insert a pseudo element and give it 100% width and height of parent. The pseudo element has a blue background in the following image.

https://i.stack.imgur.com/UdgtP.jpg

At this point, we have this CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
}

.tr: before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
}

STEP 2 : Let's rotate

First, most important : define a transform origin. The default origin is in the center of the pseudo element and we need it at the bottom left. By adding this CSS to the pseudo element :

transform-origin:0 100%; or transform-origin: left bottom;

Now we can rotate the pseudo element 45 degrees clockwise with transform : rotate(45deg);

https://i.stack.imgur.com/mEaS1.jpg

At this point, we have this CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
}

.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
    transform-origin: 0 100%;        
    transform: rotate(45deg);
}

STEP 3 : hide it

To hide the unwanted parts of the pseudo element (everything that overflows the div with the yellow border) you just need to set overflow:hidden; on the container. after removing the yellow border, you get... a TRIANGLE! :

DEMO

https://i.stack.imgur.com/M2csZ.jpg

CSS :

.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
    overflow: hidden;
}

.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: #0079C6;
    transform-origin: 0 100%;
    transform: rotate(45deg);
}

STEP 4 : go further...

As shown in the demo, you can customize the triangles :

Make them thinner or flatter by playing with skewX(). Make them point left, right or any other direction by playing with the transform orign and rotation direction. Make some reflexion with 3D transform property. Give the triangle borders Put an image inside the triangle Much more... Unleash the powers of CSS3!

Why use this technique?

Triangle can easily be responsive. You can make a triangle with border. You can maintain the boundaries of the triangle. This means that you can trigger the hover state or click event only when the cursor is inside the triangle. This can become very handy in some situations like this one where each triangle can't overlay it's neighbours so each triangle has it's own hover state. You can make some fancy effects like reflections. It will help you understand 2d and 3d transform properties.

Why not use this technique?

The main drawback is the browser compatibility, the 2d transform properties are supported by IE9+ and therefore you can't use this technique if you plan on supporting IE8. See CanIuse for more info. For some fancy effects using 3d transforms like the reflection browser support is IE10+ (see canIuse for more info). You don't need anything responsive and a plain triangle is fine for you then you should go for the border technique explained here : better browser compatibility and easier to understand thanks to the amazing posts here.

It's probably worth mentioning that the 1.41 is an approximation of √2 and the length of the hypotenuse of the triangle that you create, which is why you need (at least) that width.

I wanted to keep the answer simple but you are right, it should be mentioned @KRyan

Actually, in trying to use this, a mention of how the different widths for use with skewX were derived would be useful.

This method works better when you need 1px solid border for the triangle.

This is for the case you want to create borders for triangle, I was stuck with the borders way when I want to create this annotation box AnnotationBox

y

yunzen

Here is an animation in JSFiddle I created for demonstration.

Also see snippet below.

This is an Animated GIF made from a Screencast

https://i.stack.imgur.com/vYvqa.gif

transforms = [ {'border-left-width' :'30', 'margin-left': '70'}, {'border-bottom-width' :'80'}, {'border-right-width' :'30'}, {'border-top-width' :'0', 'margin-top': '70'}, {'width' :'0'}, {'height' :'0', 'margin-top': '120'}, {'borderLeftColor' :'transparent'}, {'borderRightColor' :'transparent'} ]; $('#a').click(function() {$('.border').trigger("click");}); (function($) { var duration = 1000 $('.border').click(function() { for ( var i=0; i < transforms.length; i++ ) { $(this) .animate(transforms[i], duration) } }).end() }(jQuery)) .border { margin: 20px 50px; width: 50px; height: 50px; border-width: 50px; border-style: solid; border-top-color: green; border-right-color: yellow; border-bottom-color: red; border-left-color: blue; cursor: pointer } Click it!

Random version

/** * Randomize array element order in-place. * Using Durstenfeld shuffle algorithm. */ function shuffleArray(array) { for (var i = array.length - 1; i > 0; i--) { var j = Math.floor(Math.random() * (i + 1)); var temp = array[i]; array[i] = array[j]; array[j] = temp; } return array; } transforms = [ {'border-left-width' :'30', 'margin-left': '70'}, {'border-bottom-width' :'80'}, {'border-right-width' :'30'}, {'border-top-width' :'0', 'margin-top': '70'}, {'width' :'0'}, {'height' :'0'}, {'borderLeftColor' :'transparent'}, {'borderRightColor' :'transparent'} ]; transforms = shuffleArray(transforms) $('#a').click(function() {$('.border').trigger("click");}); (function($) { var duration = 1000 $('.border').click(function() { for ( var i=0; i < transforms.length; i++ ) { $(this) .animate(transforms[i], duration) } }).end() }(jQuery)) .border { margin: 50px; width: 50px; height: 50px; border-width: 50px; border-style: solid; border-top-color: green; border-right-color: yellow; border-bottom-color: red; border-left-color: blue; cursor: pointer } Click it!

All at once version

$('#a').click(function() {$('.border').trigger("click");}); (function($) { var duration = 1000 $('.border').click(function() { $(this) .animate({'border-top-width': 0 , 'border-left-width': 30 , 'border-right-width': 30 , 'border-bottom-width': 80 , 'width': 0 , 'height': 0 , 'margin-left': 100, 'margin-top': 150, 'borderTopColor': 'transparent', 'borderRightColor': 'transparent', 'borderLeftColor': 'transparent'}, duration) }).end() }(jQuery)) .border { margin: 50px; width: 50px; height: 50px; border-width: 50px; border-style: solid; border-top-color: green; border-right-color: yellow; border-bottom-color: red; border-left-color: blue; cursor: pointer } Click it!

d

dippas

Lets say we have the following div:

<div id="triangle" />

Now Edit the CSS step-by-step, so you will get clear idea what is happening around

STEP 1: JSfiddle Link:

 #triangle {
        background: purple;
        width :150px;
        height:150PX;
        border-left: 50px solid black ;
        border-right: 50px solid black;
        border-bottom: 50px solid black;
        border-top: 50px solid black;
    }

This is a simple div. With a very simple CSS. So a layman can understand. Div has dimensions 150 x 150 pixels with the border 50 pixels. The image is attached:

https://i.stack.imgur.com/KdFPy.png

STEP 2: JSfiddle Link:

#triangle {
    background: purple;
    width :150px;
    height:150PX;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
}

Now I just changed the border-color of all 4 sides. The image is attached.

https://i.stack.imgur.com/fTFDv.png

STEP:3 JSfiddle Link:

#triangle {
    background: purple;
    width :0;
    height:0;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
}

Now I just changed the height & width of div from 150 pixels to zero. The image is attached

https://i.stack.imgur.com/9nByu.png

STEP 4: JSfiddle:

#triangle {
    background: purple;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
}

Now I have made all the borders transparent apart from the bottom border. The image is attached below.

https://i.stack.imgur.com/05KJk.png

STEP 5: JSfiddle Link:

#triangle {
    background: white;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
}

Now I just changed the background color to white. The image is attached.

https://i.stack.imgur.com/8dXFH.png

Hence we got the triangle we needed.

Why did you use a Nether Portal for the first step?

C

Community

And now something completely different...

Instead of using css tricks don't forget about solutions as simple as html entities:

&#9650;

Result:

▲

See: What are the HTML entities for up and down triangles?

I don't think "beaten" is an appropriate word here. The solution relies on the font metrics, so the precise positioning is rather dubious, not to mention that you have no control over the shape.

Yes, the result is not very much controllable, but this is usually the best solution when you want to use simple icon for drop-downs etc. I would place it into ::before's or ::after's content.

H

Hemerson Varela

Consider the below triangle

.triangle {
    border-bottom:15px solid #000;
    border-left:10px solid transparent;
    border-right:10px solid transparent;
    width:0;
    height:0;
}

This is what we are given:

https://i.stack.imgur.com/i3pJR.png

Why it came out in this shape? The below diagram explains the dimensions, note that 15px was used for the bottom border and 10px was used for left and right.

https://i.stack.imgur.com/NVLeK.png

It's pretty easy to make a right-angle triangle also by removing the right border.

https://i.stack.imgur.com/0Wbmg.png

A

Al Foиce ѫ

Taking it one step further, using css based on this I added arrows to my back and next buttons (yes I know its not 100% cross-browser, but slick none the less).

.triangle { width: 0; height: 0; border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 100px solid red; margin:20px auto; } .triangle-down { border-bottom:none; border-top: 100px solid red; } .triangle-left { border-left:none; border-right: 100px solid red; border-bottom: 50px solid transparent; border-top: 50px solid transparent; } .triangle-right { border-right:none; border-left: 100px solid red; border-bottom: 50px solid transparent; border-top: 50px solid transparent; } .triangle-after:after { width: 0; height: 0; border-left: 5px solid transparent; border-right: 5px solid transparent; border-bottom: 5px solid red; margin:0 5px; content:""; display:inline-block; } .triangle-after-right:after { border-right:none; border-left: 5px solid blue; border-bottom: 5px solid transparent; border-top: 5px solid transparent; } .triangle-before:before { width: 0; height: 0; border-left: 5px solid transparent; border-right: 5px solid transparent; border-bottom: 5px solid blue; margin:0 5px; content:""; display:inline-block; } .triangle-before-left:before { border-left:none; border-right: 5px solid blue; border-bottom: 5px solid transparent; border-top: 5px solid transparent; }

Back Next

how is this not cross browser? triangles should work back to IE6.

the use of :before and :after are not 100% supported.

Psuedo-elements are not supported < IE8.

m

miken32

Different approach. With linear gradient (for IE, only IE 10+). You can use any angle:

.triangle { margin: 50px auto; width: 100px; height: 100px; /* linear gradient */ background: -moz-linear-gradient(-45deg, rgba(255,0,0,0) 0%, rgba(255,0,0,0) 50%, rgba(255,0,0,1) 50%, rgba(255,0,0,1) 100%); /* FF3.6+ */ background: -webkit-gradient(linear, left top, right bottom, color-stop(0%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,1)), color-stop(100%,rgba(255,0,0,1))); /* Chrome,Safari4+ */ background: -webkit-linear-gradient(-45deg, rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%); /* Chrome10+,Safari5.1+ */ background: -o-linear-gradient(-45deg, rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%); /* Opera 11.10+ */ background: -ms-linear-gradient(-45deg, rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%); /* IE10+ */ background: linear-gradient(135deg, rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%); /* W3C */; }

Here is jsfiddle

This is a beautiful solution, but it should be noted that it's IE 10+ only.

S

Stewartside

CSS clip-path

This is something I feel this question has missed; clip-path

clip-path in a nutshell Clipping, with the clip-path property, is akin to cutting a shape (like a circle or a pentagon) from a rectangular piece of paper. The property belongs to the “CSS Masking Module Level 1” specification. The spec states, “CSS masking provides two means for partially or fully hiding portions of visual elements: masking and clipping”. Extract from Smashing Magazine

clip-path will use the element itself rather than its borders to cut the shape you specify in its parameters. It uses a super simple percentage based co-ordinate system which makes editing it very easy and means you can pick it up and create weird and wonderful shapes in a matter of minutes.

Triangle Shape Example

div { -webkit-clip-path: polygon(50% 0%, 0% 100%, 100% 100%); clip-path: polygon(50% 0%, 0% 100%, 100% 100%); background: red; width: 100px; height: 100px; }

Downside

It does have a major downside at the moment, one being it's major lack of support, only really being covered within -webkit- browsers and having no support on IE and only very partial in FireFox.

Resources

Here are some useful resources and material to help better understand clip-path and also start creating your own.

Clippy - A clip-path generator

The W3C Candidate Recommendation File

MDN clip-path documentation

clip-path Browser Support

Seems to have better support in firefox now

A

Alireza

OK, this triangle will get created because of the way that borders of the elements work together in HTML and CSS...

As we usually use 1 or 2px borders, we never notice that borders make a 45° angles to each others with the same width and if the width changes, the angle degree get changed as well, run the CSS code I created below:

.triangle { width: 100px; height: 100px; border-left: 50px solid black; border-right: 50px solid black; border-bottom: 100px solid red; }

Then in the next step, we don't have any width or height, something like this:

.triangle { width: 0; height: 0; border-left: 50px solid black; border-right: 50px solid black; border-bottom: 100px solid red; }

And now we make the left and right borders invisible to make our desirable triangle as below:

.triangle { width: 0; height: 0; border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 100px solid red; }

If you not willing to run the snippet to see the steps, I've created an image sequence to have a look at all steps in one image:

https://i.stack.imgur.com/Ws6I5.jpg

C

Community

This is an old question, but I think will worth it to share how to create an arrow using this triangle technique.

Step 1:

Lets create 2 triangles, for the second one we will use the :after pseudo class and position it just below the other:

https://i.stack.imgur.com/Ivc5C.png

.arrow{ width: 0; height: 0; border-radius: 50px; display: inline-block; position: relative; } .arrow:after{ content: ""; width: 0; height: 0; position: absolute; } .arrow-up{ border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 50px solid #333; } .arrow-up:after{ top: 5px; border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 50px solid #ccc; right: -50px; }

Step 2

Now we just have to set the predominant border color of the second triangle to the same color of the background:

https://i.stack.imgur.com/pN4A7.png

.arrow{ width: 0; height: 0; border-radius: 50px; display: inline-block; position: relative; } .arrow:after{ content: ""; width: 0; height: 0; position: absolute; } .arrow-up{ border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 50px solid #333; } .arrow-up:after{ top: 5px; border-left: 50px solid transparent; border-right: 50px solid transparent; border-bottom: 50px solid #fff; right: -50px; }

Fiddle with all the arrows:
http://jsfiddle.net/tomsarduy/r0zksgeu/

T

Temani Afif

^{If you want to apply border to the triangle read this: Create a triangle with CSS?}

Almost all the answers focus on the triangle built using border so I am going to elaborate the linear-gradient method (as started in the answer of @lima_fil).

Using a degree value like 45° will force us to respect a specific ratio of height/width in order to obtain the triangle we want and this won't be responsive:

.tri { width:100px; height:100px; background:linear-gradient(45deg, transparent 49.5%,red 50%); /*To illustrate*/ border:1px solid; } Good one

bad one

Instead of doing this we should consider predefined values of direction like to bottom, to top, etc. In this case we can obtain any kind of triangle shape while keeping it responsive.

1) Rectangle triangle

To obtain such triangle we need one linear-gradient and a diagonal direction like to bottom right, to top left, to bottom left, etc

.tri-1,.tri-2 { display:inline-block; width:100px; height:100px; background:linear-gradient(to bottom left, transparent 49.5%,red 50%); border:1px solid; animation:change 2s linear infinite alternate; } .tri-2 { background:linear-gradient(to top right, transparent 49.5%,red 50%); border:none; } @keyframes change { from { width:100px; height:100px; } to { height:50px; width:180px; } }

2) isosceles triangle

For this one we will need 2 linear-gradient like above and each one will take half the width (or the height). It's like we create a mirror image of the first triangle.

.tri { display:inline-block; width:100px; height:100px; background-image: linear-gradient(to bottom right, transparent 49.5%,red 50%), linear-gradient(to bottom left, transparent 49.5%,red 50%); background-size:50.3% 100%; /* I use a value slightly bigger than 50% to avoid having a small gap between both gradient*/ background-position:left,right; background-repeat:no-repeat; animation:change 2s linear infinite alternate; } @keyframes change { from { width:100px; height:100px; } to { height:50px; width:180px; } }

3) equilateral triangle

This one is a bit tricky to handle as we need to keep a relation between the height and width of the gradient. We will have the same triangle as above but we will make the calculation more complex in order to transform the isosceles triangle to an equilateral one.

To make it easy, we will consider that the width of our div is known and the height is big enough to be able to draw our triangle inside (height >= width).

https://i.stack.imgur.com/iFzI1.png

We have our two gradient g1 and g2, the blue line is the width of the div w and each gradient will have 50% of it (w/2) and each side of the triangle sould be equal to w. The green line is the height of both gradient hg and we can easily obtain the formula below:

(w/2)² + hg² = w² ---> hg = (sqrt(3)/2) * w ---> hg = 0.866 * w

We can rely on calc() in order to do our calculation and to obtain the needed result:

.tri { --w:100px; width:var(--w); height:100px; display:inline-block; background-image: linear-gradient(to bottom right, transparent 49.5%,red 50%), linear-gradient(to bottom left, transparent 49.5%,red 50%); background-size:calc(var(--w)/2 + 0.5px) calc(0.866 * var(--w)); background-position: left bottom,right bottom; background-repeat:no-repeat; }

Another way is to control the height of div and keep the syntax of gradient easy:

.tri { --w:100px; width:var(--w); height:calc(0.866 * var(--w)); display:inline-block; background: linear-gradient(to bottom right, transparent 49.8%,red 50%) left, linear-gradient(to bottom left, transparent 49.8%,red 50%) right; background-size:50.2% 100%; background-repeat:no-repeat; }

4) Random triangle

To obtain a random triangle, it's easy as we simply need to remove the condition of 50% of each one BUT we should keep two condition (both should have the same height and the sum of both width should be 100%).

.tri-1 { width:100px; height:100px; display:inline-block; background-image: linear-gradient(to bottom right, transparent 50%,red 0), linear-gradient(to bottom left, transparent 50%,red 0); background-size:20% 60%,80% 60%; background-position: left bottom,right bottom; background-repeat:no-repeat; }

But what if we want to define a value for each side? We simply need to do calculation again!

https://i.stack.imgur.com/POSwA.png

Let's define hg1 and hg2 as the height of our gradient (both are equal to the red line) then wg1 and wg2 as the width of our gradient (wg1 + wg2 = a). I will not going to detail the calculation but at then end we will have:

wg2 = (a²+c²-b²)/(2a)
wg1 = a - wg2
hg1 = hg2 = sqrt(b² - wg1²) = sqrt(c² - wg2²)

Now we have reached the limit of CSS as even with calc() we won't be able to implement this so we simply need to gather the final result manually and use them as fixed size:

.tri { --wg1: 20px; --wg2: 60px; --hg:30px; width:calc(var(--wg1) + var(--wg2)); height:100px; display:inline-block; background-image: linear-gradient(to bottom right, transparent 49.5%,red 50%), linear-gradient(to bottom left, transparent 49.5%,red 50%); background-size:var(--wg1) var(--hg),var(--wg2) var(--hg); background-position: left bottom,right bottom; background-repeat:no-repeat; }

Bonus

We should not forget that we can also apply rotation and/or skew and we have more option to obtain more triangle:

.tri { --wg1: 20px; --wg2: 60px; --hg:30px; width:calc(var(--wg1) + var(--wg2) - 0.5px); height:100px; display:inline-block; background-image: linear-gradient(to bottom right, transparent 49%,red 50%), linear-gradient(to bottom left, transparent 49%,red 50%); background-size:var(--wg1) var(--hg),var(--wg2) var(--hg); background-position: left bottom,right bottom; background-repeat:no-repeat; }

And of course we should keep in mind the SVG solution which can be more suitable in some situation:

svg { width:100px; height:100px; } polygon { fill:red; }

v

vsync

SASS (SCSS) triangle mixin

I wrote this to make it easier (and DRY) to automatically generate a CSS triangle:

// Triangle helper mixin (by Yair Even-Or)
// @param {Direction} $direction - either `top`, `right`, `bottom` or `left`
// @param {Color} $color [currentcolor] - Triangle color
// @param {Length} $size [1em] - Triangle size
@mixin triangle($direction, $color: currentcolor, $size: 1em) {
  $size: $size/2;
  $transparent: rgba($color, 0);
  $opposite: (top:bottom, right:left, left:right, bottom:top);

  content: '';
  display: inline-block;
  width: 0;
  height: 0;
  border: $size solid $transparent;
  border-#{map-get($opposite, $direction)}-color: $color;
  margin-#{$direction}: -$size;
}

use-case example:

span {
  @include triangle(bottom, red, 10px);
}

Playground page

Important note:
if the triangle seems pixelated in some browsers, try one of the methods described here.

D

Danziger

If you want to play around with border-size, width and height and see how those can create different shapes, try this:

D

Doml The-Bread

here is another fiddle:

.container:after {
    position: absolute;
    right: 0;
    content: "";
    margin-right:-50px;
    margin-bottom: -8px;
    border-width: 25px;
    border-style: solid;
    border-color: transparent transparent transparent #000;
    width: 0;
    height: 0;
    z-index: 10;
    -webkit-transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    bottom: 21px;
}
.container {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 150px;
    height: 80px;
    background-color: #000;
}

.containerRed {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 100px;
    height: 80px;
    background-color: red;
}

https://jsfiddle.net/qdhvdb17/

H

HelloWorldNoMore

Others have already explained this well. Let me give you an animation which will explain this quickly: http://codepen.io/chriscoyier/pen/lotjh

Here is some code for you to play with and learn the concepts.

HTML:

<html>
  <body>
    <div id="border-demo">
    </div>
  </body>
</html>

CSS:

/*border-width is border thickness*/
#border-demo {
    background: gray;
    border-color: yellow blue red green;/*top right bottom left*/
    border-style: solid;
    border-width: 25px 25px 25px 25px;/*top right bottom left*/
    height: 50px;
    width: 50px;
}

Play with this and see what happens. Set height and width to zero. Then remove top border and make left and right transparent, or just look at the code below to make a css triangle:

#border-demo {
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid blue;
}

c

coding-dude.com

I know this is an old one, but I'd like to add to this discussion that There are at least 5 different methods for creating a triangle using HTML & CSS alone.

Using borders Using linear-gradient Using conic-gradient Using transform and overflow Using clip-path

I think that all have been covered here except for method 3, using the conic-gradient, so I will share it here:

.triangle{ width: 40px; height: 40px; background: conic-gradient(at 50% 50%,transparent 135deg,green 0,green 225deg, transparent 0); }

https://i.stack.imgur.com/kh5hG.png

P

Priya Maheshwari

use clip-path: polygon(50% 0%, 100% 100%, 0% 100%); for creating easy to Triangle

<div class="triangle"></div>

.triangle{width:200px; height:200px;background:#000;clip-path: polygon(50% 0%, 100% 100%, 0% 100%);}

s

satish mallick

Try This:-

.triangle { border-color: transparent transparent red transparent; border-style: solid; border-width: 0px 200px 200px 200px; height: 0px; width: 0px; }

L

LaZza

clip-path has the best result for me - works great for divs/containers with and without fixed dimensions:

.triangleContainer{
    position: relative;
    width: 500px;
    height: 500px;
}

.triangleContainer::before{
    content: "";        
    position: absolute;
    background:blue;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    clip-path: polygon(50% 0, 0 100%, 100% 100%);
}

not sure what is wrong with my suggestion. I just used it on another project and works a treat - happy to make a triangle on request with a variation on this technique for anyone interested!

The problem was that the OP wanted to know how it works, what is the reason doing this or that results in a triangle? not how to do it.

How do CSS triangles work?

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