Draw the First Arc

What happens here

With the nails securely in place, position the framing square so its two arms are resting firmly against each nail, and the inside corner of the square touches the workpiece. Take your marking tool (a pen is used in the video) and place its tip directly into the inside corner of the framing square. While maintaining constant pressure, both on the square against the nails and on the marking tool against the square's corner, carefully pivot the framing square around the two nails. This motion will draw the first semi-circular arc.

Goal: Create the first half of the circle by pivoting the framing square.