I would also vote for getting a 12.5"X12.5" square up ruler. Wait until quilt rulers go half off at JoAnn's or use a coupon and get it online. Although you certainly can use a generic square cut from plexiglass or other material, it really is much easier to use if there are ruler markings, especially a true diagonal. With the 12.5" version you can square everything from that size on down. You can also use it for other things, like squaring up fabric before cutting strips. I finally broke down and got the 20.5" one when it was on sale. I started doing 16" blocks and it made my life much easier!
And remember, as you collect rulers, it's wise to get all the same brand as the measurements can vary just a bit.
Last edited by PaperPrincess; 04-01-2014 at 08:54 AM.