Bloc Loc Half Square Triangle 2.5" Acrylic Ruler - HST-2.5Bloc Loc's Half Square Triangle ruler 2 1 2" x 2 1 2" locks onto the seams for instant ruler placement, control so that the seams end in each corner and trims 90% faster than a flat ruler. Ruler rock is gone, too! Also trims quarter square triangles, birds in the air, y blocks, LeMoyne Star, double windmill, hidden square units and trims in 1 8" increments.