Steel tubing blocks for rack building. The three main grooves are 1/4, 5/16 and 3/8 inch. The 1/4 inch holes in the center of the grooves allow you to accurately drill the center-line of the tubes for water bottle bosses. You use the grooves on the side to guide 90 degree miters, or use them to clamp smaller pieces of tubing. The flat edge is useful as a saw guide.