Originally Posted by dunster

Even if there were no need for a "scant" 1/4", being consistent is not enough when you are dealing with more complicated blocks. Suppose you decide to sew 1/2" seams instead of 1/4". If you make a 9-patch it looks fine. If you make a 4-patch it also looks fine. But now take a 9-patch and a 4-patch that were supposed to be the same finished size and try to put them together. It won't work, because there are 2 seams going down the 9-patch and only one seam going down the 2-patch! This is an exaggeration, but the theory holds even if your seams are only 1/16" too large or too small.

The lesson to take from this is that you need to measure your block as it is formed to be sure your units are measuring correctly. The seam size needs to be adjusted so that the *finished* patch is the right size.