Altitude of a triangle

The perpendicular from a vertex to the opposite side
Try this Drag the orange dots on each vertex to reshape the triangle. Note the position of the altitude as you drag.

The altitude of a triangle is a line from a vertex to the opposite side, that is perpendicular to that side, as shown in the animation above. A triangle therefore has three possible altitudes. The altitude is the shortest distance from a vertex to its opposite side.

The word 'altitude' is used in two subtly different ways:

It can be outside the triangle

In most cases the altitude of the triangle is inside the triangle, like this:

Angles B, C are both acute
However, if one of the angles opposite the chosen vertex is obtuse, then it will lie outside the triangle, as below. The angle ACB is opposite the chosen vertex A, and is obtuse (greater than 90°).
Angle C is obtuse
The altitude meets the extended base BC of the triangle at right angles.

In the animation at the top of the page, drag the point A to the extreme left or right to see this.


It turns out that in any triangle, the three altitudes always intersect at a single point, which is called the orthocenter of the triangle. For more on this, see Orthocenter of a triangle.


The following two pages demonstrate how to construct the altitude of a triangle with compass and straightedge.

Things to try

In the animation at the top of the page:

Other triangle topics


Perimeter / Area

Triangle types

Triangle centers

Congruence and Similarity

Solving triangles

Triangle quizzes and exercises