VertexA corner where two sides meet. A triangle has three, and it is named by them, as in triangle ABC.
SideOne of the three straight segments that make the triangle.
BaseThe side the height is measured to. Usually the one drawn at the bottom.
Height (altitude)Segment from a vertex meeting the opposite side at a right angle. Area is half base times height.
Interior angleThe angle inside the triangle at a vertex. The three add up to 180°.
Right triangle: 3 3
HypotenuseThe side opposite the right angle, always the longest side.
LegOne of the two sides that form the right angle. For legs a and b, a² + b² = c².
Right angleAn angle of exactly 90°, marked with a small square.
Other 1
MedianA segment from a vertex to the midpoint of the opposite side. It splits the triangle into two equal areas.
How this list was compiled: Read 2026-09-26. (1) OpenStax Prealgebra 2e, 9.3 Use Properties of Angles, Triangles, and the Pythagorean Theorem, https://openstax.org/books/prealgebra-2e/pages/9-3-use-properties-of-angles-triangles-and-the-pythagorean-theorem : triangles are named by their vertices; each side is labeled to match the opposite vertex; the angles of a triangle add up to 180°; in a right triangle the side opposite the right angle is the hypotenuse and the other two sides are the legs; the Pythagorean Theorem. (2) OpenStax Prealgebra 2e, 9.4 Use Properties of Rectangles, Triangles, and Trapezoids, https://openstax.org/books/prealgebra-2e/pages/9-4-use-properties-of-rectangles-triangles-and-trapezoids : the base is one side, usually at the bottom; the height connects the base to the opposite vertex at a 90° angle; area is one-half base times height. (3) Wikipedia, Triangle, https://en.wikipedia.org/wiki/Triangle : an altitude runs through a vertex perpendicular to the opposite side, its base; a median joins a vertex to the midpoint of the opposite side and divides the triangle into two equal areas; an exterior angle is supplementary to the interior angle; exterior angle theorem. (4) Wikipedia, Right triangle, https://en.wikipedia.org/wiki/Right_triangle : hypotenuse opposite the right angle, legs adjacent to it. (5) Merriam-Webster, median, https://www.merriam-webster.com/dictionary/median : a line from a vertex of a triangle to the midpoint of the opposite side. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
The parts of a triangle are its three vertices, three sides and three angles, plus the lines drawn inside it. The left triangle shows those; the right triangle adds the hypotenuse, legs and right angle.
Read the full guide
Base and height
Any side can be the base; it is usually the one drawn at the bottom. The height is the line from the opposite vertex that meets the base at a right angle, marked by the small square. Area is half the base times the height.
Median or height?
Both start at a vertex. The height meets the opposite side at a right angle; the median goes to its midpoint, marked by the two ticks, and cuts the triangle into two halves of equal area.
The three interior angles always add up to 180°, so two of them give you the third.
Questions and answers
What is the hypotenuse of a triangle?
The side opposite the right angle in a right triangle. It is always the longest side. The two sides that form the right angle are the legs, and a² + b² = c² links them.
How many vertices does a triangle have?
Three, one at each corner. A triangle is named by them: triangle ABC has vertices A, B and C, and each side is named after the vertex opposite it.
Is the height of a triangle always inside it?
No. In a triangle with an angle over 90°, the height from some vertices falls outside, meeting the base extended beyond the triangle.