The Math of GR
Part 3
For convenience, make the Z rod go straight up and down, so that the X-Y rods are horizontal to the ground.

Have someone hold one end of a string right where the ball is. Take the other end of the strong and touch it to the Z rod, which we will call the Z axis. Make sure of two things: (1) the string is straight and (2) the line that the string makes with the axis forms a 90 degree angle. Mark the place where the string hits the Z axis with orange sharpie.

Do this with the Y and also the X axis. Therefore, you now have an orange mark on all three rods. Next, pick a system of units. We will use INCHES. Mark each rod in one inch intervals with a black sharpie, starting one inch away from the ORIGIN, where all the rods connect.