Prism99

What counts is how big the pieces are *after they have been sewn together*. Cut 3 pieces of fabric 2.5" wide and sew them together so you have 3 pieces side-by-side. Press. Measure the width of the pieces. Here is where you want an exact 1/4" off each piece you have cut, so the width of the pieces sewn together should be exactly 6.5" wide. If you have used an exact 1/4" seam, you will find that the width is less than 6.5". This is because turn-of-the-cloth (and thread size) take up a little of the fabric.

This is why you cut accurately, and then sew with a scant 1/4" seam, in order to get a perfect 1/4" taken off each piece after it is sewn to other pieces. The "scant" depends a lot on the thread you use and how accurately you iron. The thicker the thread used to sew the seam, the more "scant" your seam needs to be to allow for turn-of-the-cloth when you iron.

Edit: Patterns are drafted with accurate 1/4" in the finished pieces. This is what we try to achieve with sewing. It's just that if you sew a full 1/4" seam and then iron the pieces flat, you will not achieve the size that the pattern was drafted.

It's true that some patterns allow you to use any size seam and the pattern will still work -- only the size will be slightly different. However, more complex patterns really require that accurate *finished* 1/4" seam in order to have all the seams match.