Originally Posted by MsHeirloom

My understanding is that the "scant" 1/4" seam allowance works because the "turn of the cloth"- fold takes up a small amount. If the seam allowance is slightly less than 1/4" the finished block will be closer to the actual size block one is trying to accomplish!

Originally Posted by bearisgray

even though I think I cut "exactly" - when I measure my 2-1/2 inch strip - it is actually about 2-9/32 inches wide - about a rotary blade's thickness wider than the template.
the fabric has to turn and accommodate the thread - and the result would be less than 4.5 inches
I think the place to start is to measure the size of one's cut pieces. A couple of threads here and there can make a difference in the finished size of the unit.

Originally Posted by dunster

Being consistent is sometimes enough, but not always. If you're doing a quilt consisting entirely of 9-patch blocks, for instance, you will be fine with any size seam allowance. However once you start mixing in angles or blocks that have different numbers of seams, you need to patches that measure correctly in order to have the whole thing come together accurately. Quilt patterns are designed with a 1/4" seam in mind, but the theory underlying this is that you're living in a 2-dimensional world. However, fabric and thread are not 2-dimensional; they have some thickness. When you take two patches that are 2" square and sew them together with a seam that is exactly 1/4" you hope to wind up with a rectangle that is exactly 2" x 3.5" (losing 1/4" of each 2" square in the seam). However you are going to wind up instead with a rectangle that is 2" x something less than 3.5". How much less will depend on the thickness of your fabric and thread, because these things cause some of the fabric to be "lost" in the seam. This is why the "scant" 1/4" is important, and how scant depends on the thickness of your fabric and thread and how well you can press the seam flat (which again depends on the fabric and thread).

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.

Originally Posted by maviskw

2 9/32 inch is actually closer to 2 1/4 inch. If you are working with 2 1/2 inches, that is 2 16/32 inches. So a 32nd inch larger is 2 17/32.
You would think thread doesn't take up any space, but look how it piles up on a spool. And folding takes a little space, too. I aim for pieces a little larger and trim segments of the block as they are made. If I'm making a 12 1/2 in. pin wheel, I need 4 HSTs. After they are sewn, I trim them to exactly 4 1/2 inches. Then one more seam each way, and I have a 12 1/2 inch block.
Trimming smaller segments of the block makes things come together accurately.
I know some of you can do your seams perfectly the first time, but not this chick.