Oh also...if you aren't familiar with Quilterscache here's a link to the 12" blocks
http://www.quilterscache.com/BlocksbySizeC.html
Special note --- they say 12" because that is their finished size .... The blocks you make should be 12 1/2" unfinished. Please don't trim your blocks down to 12" ...
Not to insult any one knowing or not knowing but it can be confusing and I'd rather be upfront and make sure we all are on the same page. "Finished" size is what your block would be after it is in a quilt. "Unfinished" is the block just laying on the table waiting to be included in it's permanent home in a quilt top.
Also if using a pattern from Quilterscache, I'd suggest using a "SCANT" 1/4" seam or you might have a size problem. Some of them are cut pretty tight.