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How do CSS triangles work?

Asked 11 years ago
Viewed 201.43 k times

There're plenty of different CSS shapes over at CSS Tricks - Shapes of CSS and I'm particularly puzzled with a triangle:

CSS Triangle

#triangle-up {
  width: 0;
  height: 0;
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
}
<div id="triangle-up"></div>

How and why does it work?

asked 11 years ago

Correct Answer

CSS Triangles: A Tragedy in Five Acts

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

borders meet at 45 degree angles, content in middle

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

no top border

Then you give it a width of 0...

no width

...and a height of 0...

no height either

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

transparent side borders

That results in a triangle.

answered 6 years ago

Other Answer

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

Border example

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

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.

answered 11 years ago