Bloc Loc's half square triangle rulers make the tough job of squaring up easy and fast. The groove slides along the seam to trim smaller units and allows quilters to rotate the block without lifting the ruler. Set includes: 2.5' and 4.5', rulers. have a groove to lock the ruler into place on your seam make the tough job of squaring up easy and fast Easy to read numbers and grid lines in 1/2' increments Acrylic