A triangle ABC is shown. Drag any vertex. From each vertex an altitude is drawn — the line straight down to the opposite side, meeting it at a right angle. All three altitudes pass through a single point, the orthocenter H. For an acute triangle H is inside; for a right triangle it sits exactly on the right-angle vertex; for an obtuse triangle it lies outside. Toggles show the altitudes, the feet with right-angle marks, and extend the sides when feet fall outside. An info button opens a drawer explaining the concept.
The three altitudes meet at one point — the orthocenter H
Each altitude runs from a vertex perpendicular to the opposite side. Drag a vertex obtuse and H slides outside the triangle.