Half-Square Triangle Ruler Set #5Bloc Locs Half Square Triangle Set #5 contains 4 1 2, 5 1 2 & 6 1 2 rulers that lock onto the seam for instant ruler placement, control so that the seam ends 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 much, much more. HST rulers are multi sized and trim in 1 8 increments.