Triangle their internal angles, measured here in degrees. A triangle that has two angles with the same measure also has two sides with the same length, and therefore it is an isosceles triangle. It follows that in a triangle where all angles have the same measure, all three sides have the same length, and such a triangle is therefore equilateral. Triangles are assumed to be two-dimensional plane figures, unless the context provides otherwise (see Non-planar triangles, below). In rigorous treatments, a triangle is therefore called a "2-simplex" (see also Polytope). Elementary facts about triangles were presented by Euclid in books 1–4
Arrow (symbol) Arrow (symbol) An arrow is a graphical symbol such as ← or →, used to point or indicate direction, being in its simplest form a line segment with a triangle affixed to one end, and in more complex forms a representation of an actual arrow (e.g. ➵ U+27B5). The direction indicated by an arrow is the one along the length of the line towards the end capped by a triangle. The typographical symbol developed in the 18th century as an abstraction from arrow projectiles. Its use is comparable to that of the older (medieval) manicule (pointing hand, ☞). Also comparable