Get perfect half square triangles every time with these durable acrylic rulers. With 1/2" markings and a centered "channel", cutting half square triangles has never been simpler! Bloc Loc® Rulers work with any method for making half-square triangles, so whether you prefer using paper or sewing triangles together, Bloc Loc will help you square-up perfectly each and every time. There’s no ruler rock or slipping either. patent pending channel technology locks onto your seam allowance which makes squaring up so much easier and safer because you won’t need to put hand pressure on the ruler to make it work. Simply align the channel over the seam and trim.