The triangle on the left is called Right Triangle ABC. A,B,C correspond to the points on the triangle that define the angles within the included sides of the triangle.

If there is no confusion as to which angle is being referenced, each angle can be called by its point name, i.e. Angle A, Angle B, Angle C.

If there is confusion because several angles emanate from the same point, then each angle is called by it's full name which includes all 3 points of the line segments forming it. Angle A can be called angle BAC or CAB. The vertex of the angle is the middle letter which is the intersection point of the two line segments forming it. If line segment AB were extended to form the larger line segment AD passing through B, then angle A could also be called angle DAC or CAD as well as BAC or CAB.

The sides in this triangle are labeled a,b,c. Side a is the same as line segment CB or BC. Side b is the same as line segment AC or CA. Side c is the same as line segment AB or BA.

This is a right triangle because one of its angles is a right angle which means the angle measures 90 degrees.

The sum of the other 2 angles is also 90 degrees since the sum of the interior angles of a triangle is always equal to 180 degrees.

Since the sum of the other 2 angles is 90 degrees, those 2 angles are complements of each other.