Bloc Loc's Half-Square Triangle 5 1/2" x 5 1/2" ruler locks onto the seam for instant ruler placement and control so that the seam ends in each corner and trims 90% faster than flat ruler. Ruler rock is gont, too! Also trims quarter-square triangles, birds in the aire, y-blocks, LeMoyne Star, double windmill, hindden squares and much, much more! HST ruler is multi-sized and trims in 1/8" increments.