Originally Posted by
Basketman
I still don't get it about a 1/4" seam and a scant quarter....my logic says that if everything is cut correctly and sewn the same and everything is off a scant amount, it is universally off in all directions and even if wrong...everything should match? I get if you are hit and miss in sewing exactly, then nothing will line up but this question keeps popping up and every time I am confused. I have never tested any of my seams to see if they are "off" and have never had a problem with matching up blocks to their neighbor, but then again I am only 6 quilts to the good and maybe just beginner's luck?
On some blocks, you're right. If you have points, as Onebyone says, you will often have trouble if your blocks don't measure correctly. But even in "simple" blocks that don't have points, you may wind up with problems.
Suppose you are putting together a quilt where every other block is a plain (unpieced) block that measures 9" finished, 9.5" unfinished. You cut it accurately. Now your second block consists of 9 strips that measure 1.5"x9.5" unfinished, and finish at 1"x9". You cut them accurately. But when you sew them together, you sew just a bit too wide a seam. Let's guess that the seam is only 1/32" too wide. That means that you've used up 1/16" extra (counting top and bottom fabrics) of your fabric in the seam, fabric that should have been in the block. When you finish sewing all 9 strips together, you've made 8 seams, and each one was 1/16" too big - so your block will be 1/2" too small! (8*1/16=8/16=1/2") Now your block is going to finish at 8.5"x9" - it isn't even square! And you want to sew it to blocks that finish at 9x9". Do you see the problem? Even if you were only using the strippy blocks in your quilt, you wouldn't be able turn them at 90 degree angles, because they're not square!
This example is based on your fabric and thread taking up absolutely no space when you press your seams. But they usually do take up some space - which is why the scant 1/4" comes into play, and why you need to measure your block, not your seams, as Onebyone's original post suggests.