I recently tested the Bloc_Loc ruler. The staff person at my local quilt shop told me it was for squaring up half square triangles.
I always make these by using squares that are cut slightly larger than the size I need and I 'square them up' so they are perfect every time. I usually use my 6" square ruler to trim them square.
The Bloc_Loc has a diagonal groove on the underside, designed to 'lock' into place along the seam line. When I first put the ruler on my half square triangle I knew something was wrong.
If I had trimmed with the ruler in that spot I would definitely be trimming too much!
I had to turn the ruler around, be sure that it was 'locked' and that the block size I wanted was inside the fabric edges.
I trimmed the right side and the top,
then I turned the block around and lined up the cut edges with the measurements on the ruler and made my final two cuts. These are perfectly trimmed.
The only drawback I see with this ruler is that there are no instructions with the tool. If you go to their website (www.blocloc.com) you can see exactly how to trim the blocks.
The key part that I didn't know until I watched the video is that the Bloc_Loc logo on the ruler should always be placed on the lower side. That is the one that does not have the seam allowances underneath. Once I knew that it was very easy to position the ruler and trim my blocks.
The video also shows how to trim quarter-square triangles accurately. This is definitely my new 'go to ruler' for trimming half square triangles. I used the 6-1/2" ruler but they even have a 12-1/2" BlocLoc ruler for larger blocks.