Beating a Dead Horse
 

Please indulge me with this question. I understand a scant 1/4 inch. But if it's different from a regular 1/4, it would stand to reason that the blocks would measure differently. If you sew 3 two inch strips together, the center strip should measure 1 1/2 inches. What would it measure with a scant 1/4 inch? I don't get it.

I just always try to do the quarter inch. Scant tells me that it's 2-3 threads from 1/4 inch on either side.

Watch this youtube video which explains it very well

http://www.youtube.com/watch?v=aBoLPslLxTs

When a pattern calls for a scant 1/4" or a 1/4" they mean whatever size seam you need to achieve the result you mention in your post. What some don't understand is it's a moving target. You can mark your machine, or use a specific foot, but if you change the thickness of your thread or fabric, you may have to adjust your markings. You give a great example on how to check, here's another one:

http://www.quiltingboard.com/tutorials-f10/achieving-accurate-1-4%22-seam-allowance-t89997.html

btw, you are not beating a dead horse! each time this question is posted, I'm sure a light bulb goes on for somebody out there!

Paper Princess explained it well. In a purely theoretical world, sewing at the 1/4" line would be perfect. But because fabric and thread have thickness, we lose a small width of fabric in the fold. Since we don't want to lose that small width of fabric in the part that winds up in the quilt top, where pieces need to match up in size, we adjust by taking a slightly smaller seam.

You have the right idea -- blocks do measure differently -- but you are thinking about it in the opposite way. Sewing a regular 1/4" seam allowance will result in blocks that are a different size. A scant 1/4" seam allowance is needed to achieve the correct finished size. The video posted above demonstrates the difference in finished blocks very well.

Thanks. I loved the video. It will assure me of a scant seam. I think I will now test each new project. It.s really simple and will not be guess work. I use bottom line and aurafill.

In other words...forget "scant". Just do the 3 strip check and if your centre strips
measures 1 1/2 inches then you are good to go. If not, adjust your needle position
accordingly.

I have always been annoyed with patterns that say use a scant 1/4 inch seam. I have a 1/4 inch foot and I use that no matter what the pattern says. If you sew on the same machine and are consistent, you should be okay.