Def. 21. Further, of trilateral figures, a right-angled triangle is that which has a right angle, an obtuse-angled triangle that which has an obtuse angle, and an acute-angled triangle that which has its three angles acute.
Definition 20 classifies triangles by their symmetries, while definition 21 classifies them by the kinds of angles they contain.
The scalene triangle C has no symmetries, but the isosceles triangle B has a bilateral symmetry. The equilateral triangle A not only has three bilateral symmetries, but also has 120°-rotational symmetries.
As defined by Euclid, an equilateral triangle is not to be considered as an isosceles triangle, but in modern terminology, it is usually the case that equilateral triangles are included among the isosceles triangles, that is, it is only required that at least two sides be equal in order for a triangle to be isosceles. Generally speaking, modern definitions are inclusive whereas Euclid's definitions are usually exclusive.
Equilateral triangles are constructed in the very first proposition of the Elements, I.1. An alternate characterization of isosceles triangles, namely that their base angles are equal, is demonstrated in propositions I.5 and I.6.
Since triangle D has a right angle, it is a right triangle. Proposition I.17 states that the sum of any two angles in a triangle is less than two right angles, therefore, no triangle can contain more than one right angle. Furthermore, there can be at most one obtuse angle, and a right angle and an obtuse angle cannot occur in the same triangle.
Triangle E is an obtuse triangle since it has an obtuse angle, while triangle F is an acute triangle since all its angles are acute.