Think you 1/4" seam is 1/4"?
 

I was sure my 1/4" seam was exactly 1/4" . I did the sew 2 1/2" blocks and measure type test. I found this test by Judy Martin and I tried it. I was shocked my 1/4" was off. I had a little problem with block pieces fitting but didn't think it was because of my 1/4". Cut nine
1 1/2" squares and sew together end to end. Press seems to one side. Cut a 9 1/2" x 1 1/2" rectangle. Place sewn strip face to face to the rectangle strip. Your sewn strip should be exactly the length of the rectangle. Don't measure the length with a ruler, measure it to the strip of fabric. Adjust your sewing seam until the sewn strip is a perfect match.

thanks for the tip

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?

If your units/blocks/quilts end up being the size you are expecting them to be - all is well.

You can continue to do as you have been doing.

Actually, you are right. My original quilting teacher used to cal this your "personal, private measurement" or PPM. The only time it really affects you is when you are trading blocks with another sewer.
My own PPM is one click to the left of center on the needle position selectors of my 1970s and my 1990s mechanical Berninas, my preferred piecing machines. It's a bit less than 1/4 inch but I really don't care! I use the attached technique to be sure each seam I sew is accurate all the way down a long strip. With it I don't have to eyeball the edge of the pressure foot and can sew a long strip or any seam very quickly. And very accurately!

You make yours your way; I'll make mine my way!

Jan in VA

The 1/4" is crucial for not losing points. Ever have to fudge the seam to keep a point on a flying geese? Trouble squaring up blocks and keeping 1/4" seam allowance past a point? My quilting life is easier having a true 1/4" seam throughout the quilt. How do you keep points if your seam allowance is consistent but slightly more then 1/4" and you cut the block pieces the size it states in the pattern? It won't work for me at all. A slightly less then 1/4" works fine but the seam will be narrower.

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.

I find I lose a bit with the hump that is created when I press my seams to one side. You just have to find out what works best for you.

I don't have a 1/4 inch foot, but my straight stitch foot is just short of 1/4 inch so this is the foot I use for all of my piecing and I have not had any problems with my blocks matching up.