Geometric construction Class 9 has solutions for building a 90-degree angle. First, draw a line segment and call it "ab." Now, take the compass and keep it in the middle. Cut a small arc on the line ab. Give it the letter m. Now, cut a small arc on the first arc you drew, using the same radius as before and m as the centre. Using the same radius and centre (n), cut a small arc at point (p) on the last arc. Again, draw an arc with its centre at n and its radius greater than np. Then, using the same radius, draw the arc that will cut the first arc at q. Now join aq. It is the angle that is 90 degrees.