Prism99

Here's the thing. Patterns are actually created using perfect 1/4" seams. For example, a finished 9-patch block made with 3" squares means the finished block will measure exactly 9" square. Each patch gets an exact 1/4" seam added to it. So, patterns are made with exact measurements.

The problem is that the thread you use to sew a seam, plus the little bit of fabric that is in the roll of the seam when you press it, eat up tiny bits of that perfect 1/4" seam. If you sew perfect 1/4" seams for that 9-patch block, meaning your needle enters the fabric exactly 1/4" from the edge of the pieces, you will end up with a finished block that measures something less than a perfect 9". This is because the thread used to sew the seams, and even more the "turn of the cloth" from each seam being ironed, both eat up a little bit of that perfect 1/4".

This does not matter if you are sewing only strips or squares, and it does not matter much if the blocks are simple blocks. With many blocks, such as rail fence, you simply cut your blocks so they end up square. Where you start to run into trouble is with triangles, complicated blocks, and blocks with many pieces. Those slight differences in the finished measurements start adding up, and can create impossible matching conditions.

What's important is to test your seam allowances. The test is not how scant you sew, but rather how accurate the *finished* piece is. The usual way to test is to sew three 2.5" strips together, press, then measure. If your seam is correct, the width should measure exactly 6.5". If it's smaller than that, you need to make your seam allowance smaller. If the width is bigger than that, you need to make your seam allowance bigger.

The importance is in the finished measurements, not on how exact or scant your seam allowance is!

Edit: Just want to add that you don't even have to bother with measuring for simple patterns, such as rail fence. All that is important for those simpler patterns is consistency with whatever seam allowance you use. Achieving consistency is the topic for another thread!