Originally Posted by PatriceJ

if you have exactly 1/4" seam allowance around the "finished" square, you should sew other pieces to them using what's called a "scant quarter inch". a scant quarter inch is really 3/16" of an inch.

the reason for using a scant QI is that the thickness of the thread and the fold back "eats" about a 16th of an inch at the seam.