Mathematics for IGCSE & O level - Geometrical Constructions (Section 2 - No. 6)
If you are measuring an angle and the zero line of your protractor is aligned with one of the angle's arms, where should the center of the protractor be?
At the end of the arm.
At the vertex of the angle.
At any point along the other arm.
At the edge of the protractor.
Paliwanag
The center of the protractor must be at the point where the two lines forming the angle meet, the vertex.