I solved a lot of my 1/4" problems by using a thin thread for piecing. I learned that 50wt 3 ply is too thick for piecing when the seam is pressed to one side. I use 50wt 2 ply like Aurifil for the top, with a size 80/12 topstitch needle. For the bobbin I use size 60 or 70 poly. Most thin threads will be poly as it is stronger then cotton. 50wt 3 ply is great for machine quilting for the stitches to show.

a scant 1/4" is a couple of threads over.
I don't think there's a specific foot for it on any machine
On my Jem Platinum I can set the stitch for a scant 1/4" - which is one of the reasons I bought it (plus it has a needle down function).
Do what LindaR said.

The difference is not much. But it adds up. You can get a couple of tools to help with the scant seam allowance from Marci Baker.

This tool, http://shop.online-quilting.com/shop...&category_id=2 will help you find the scant quarter on your machine.

This tool, http://shop.online-quilting.com/shop...&category_id=2 will mark that edge so you know where it is.

I sew with 1/4" seams, unless I'm making a HST (which will then be trimmed to size, if needed).

I agree with sewmary. I just don't pay attention to "scant".

As long as we are consistant, I think if we are making blocks for ourselves it isn't important as long as the finished pattern looks good.
If we are making blocks for an exchange we have to be a little more dilligent so everyone's blocks fit together.

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!

For those who say that the seam doesn't matter: As long as your blocks don't have to be a certain size and as long as there are no points along the outer edges you may be correct. Many blocks don't matter.

Having participated in some swaps where size mattered the regular seam on some blocks did not give me the finished size. So I use the scant 1/4" to make the size work. Also when a block has HSTs, it may require less than a standard seam so you don't cut the points off.

I've sewn many star blocks, with points along the outer edges, using an exact 1/4" seam. Points end up right where they're supposed to be, 1/4" from the raw edge (stitching line). Maybe the "trick" is in pressing, correctly.