It can get confusing, when blocks sizes are mentioned, we need to be aware of the terms finished sized or unfinished. Most blocks unfinished sizes will be 1/2" larger to accomodate for the 1/4" seam allowance on all sides of it. So you would want a ruler that allowance you to square up your block to 12 1/2" :D:D:D
If you measure these blocks when your quilt is done, they will measure 12".