An altitude of a triangle is a segment from a vertex of the triangle, perpendicular to the side opposite that vertex of the triangle.

Since all triangles have three vertices and three opposite sides, all triangles have three altitudes.

The three altitudes of any triangle (or lines containing the altitudes) intersect at a common location called the orthocentre.

The orthocentre occurs inside a triangle if and only if the triangle is an acute triangle.

The orthocentre is coincidental with the vertex where the right angle occurs if and only if the triangle is a right triangle.

The orthocentre occurs outside a triangle if and only if the triangle is an obtuse triangle.

To verify these statements visually, please select "SHOW ALL ALTITUDES" in the demonstration applet above, then drag the vertices to create acute, right and obtuse triangles.