With the rings to hand, which I intended to use to mount the taillights behind, I could determine the distance between the upper and lower horizontal tubes of the nerf bar. The rings are 3 1/2 inches ID and 4 1/2 inches OD, making the tubes 4 1/2 inches apart. Once I'd made the first bend and could determine where the outer end of the nerf bar would be, I measured to the start of the bend (where the steel rule is). Knowing the width of my pickup bed and the length of each bend to the point at the ends of the nerf bar, I could determine the length of the straight section between the bends, and hence mark where the second bend should start. Got that? The instructions provide thorough details on how to measure where to start the bends.