Right angle (geometry)

Diagram showing the definition of a right angle. The green parts are not part of the construction but show that the angles are both 90 degrees and equal to one another

In Euclidean geometry a right angle is an angle that the angle made between the arms of the angle is equal to the angle adjacent to it. This angle is ninety degrees.

The right angle bisects the angle of the line into two equal parts.

The right angle is demonstrated:

Given a line DC with point B lying on it
Take B as the vertex of angle ABC
If the angle ABC equals the angle ABD 
then angle ABC is a right angle, 
and so is angle ABD