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.

The important thing is to get the finished unit to be the size you want it to be.

Some of the variables - along with the seam allowance are:

Cut size of the pieces - there can be variations - it is a good idea to use the same ruler or cutting system for the whole project
Was the fabric washed before cutting - some fabrics shrink a bit when they are steam pressed
Fabric thickness
Thread thickness

The moving target explanation is one of the best - and most comprehensive answes - that i've seen.

The three-strip test unit is worth the time if super accuracy is important.

Thanks JanieW for the link to that video. It explains it really well because I have always wondered about the 1/4" vs scant 1/4" too. Now I know.:)

Remember- it is not the size of the seam allowance that is important, but the size of the block!
Sew whatever it takes to get the block correct.

Or, just go with whatever you get and do not change anything and accept what your final dimensions are on your whole piece, quilt, item. If not, then drive yourself nuts chasing the elusive scant 1/4" seam, or obsessing over it.

Consistent seaming, no matter what the seam size, works for many, many quilt patterns. As long as all of the pieces going together are of similar types -- say, all strips and squares, or all half-square triangles -- there is no problem.

The scant 1/4" seam becomes important with more complex blocks -- for example, those that mix squares with triangles, as in some star blocks. At that point if you are not using a scant 1/4" seam, you will start noticing star tips being cut off. The scant seam is also important when sewing complex blocks to each other, to make sure they match up with each other.

It should measure 1 1/2.Because,the scant makes up for the space in the seam the fabrics take up.Another way to try it out,is,cut 2 squares the same size.Put right sides together.Sew E X A C T U A L L Y on the diagonal.Now cut 1/4in. beyond the stitchinq.And press the 1/2 square triangle open.See how your seam is thrown off by the thickness of the fabric.You need to sew just a scant to the right of the diagonal to get it exactly on the diagonal because of the thickness of the fabric.