Plastic-3/4x3/4 is used on a 3/4x3/4 Square tube, which has 3/4x3/4 on the top and 1/2x1/2 on the bottom. The bottom 1/2x1/2 is to fit into the 3/4x3/4 Square tube. Plastic-1x1 is used on a 1x1 Square tube, which has 1x1 on the top and 3/4x3/4 on the bottom. The bottom 3/4x3/4 is to fit into the 1x1 Square tube.