Typically, when you piece a quilt, you'll use a quarter-inch seam allowance-- also known as a "quarter inch seam."
A seam allowance tells you how far away to sew from the edge of your fabric. A quarter-inch seam is a seam sewn-- you guessed it-- 1/4 inch away from the fabric edge.
So, what's a scant quarter inch seam?
A scant quarter inch seam is any seam with an allowance just under 1/4". There's no precise measurement here, and this term can mean slightly different things to different quilters. But the key is that you are always sewing a consistent seam, and that is somewhere under the standard 1/4" seam allowance.
A regular quarter inch seam is shown on the left, and the stitching falls directly under the line marking 1/4". A scant quarter inch seam is shown on the right, and the stitching for this one falls just under 1/4". Some scant quarter inch seams leave even less seam allowance.
Some quilters sew their "scant quarter inch" closer to the quarter inch mark. I fall into this camp-- take a look at my 1/4 inch seam above.
Others sew really small seams, that are closer to 1/8". This can give you more trimming room... but remember, the more fabric you have in your seam allowance, the stronger your seam! If your seam allowance is getting really small, lower your stitch length to 1.5 or 1.8 mm to make your stitching a bit stronger.
When do I use a scant quarter inch seam?
Use a scant 1/4" seam when the pattern calls for it, especially if the pattern designer calls it out as required.
Or, use it when you're sewing blocks "to size" - when your pattern or block calculations don't include extra fabric for trimming. Sewing with the scant 1/4" seam will give you a little extra wiggle room to square up and trim.
For example, a scant 1/4" seam is recommended for the Petal Points quilt, because the pattern is written "to size".
How do I sew a scant quarter inch seam?
Try out your scant 1/4" seam on some scrap fabric before moving on to your actual quilt. This might seem like an extra step, but it will actually save you a ton of unpicking and wasted fabric down the line!
Fabric lined up under the presser foot to create a regular 1/4" seam (left) and scant 1/4" seam (right). Note how similar these look - the fabric is only moved over by a millimeter.
To practice your scant 1/4" seam, start by lining up two straight-edged scraps.
Sew your normal 1/4" seam along the matched side, using your quarter-inch foot to guide you.
Now, take that same set of scraps and position it back under your sewing machine foot. Scoot the fabric over just a millimeter or two - so the needle falls just below 1/4" from the edge of the fabric.
Sew this seam - the scant 1/4" seam - being careful to keep your seam consistent. Pay attention to where the edge of the fabric meets your sewing machine foot as you feed it in.
Fabric scraps with regular and scant 1/4" seams sewn next to each other.
Ta-da! There's your scant 1/4" seam. Now that it's next to your regular 1/4" seam, you can clearly see the difference between the two.
Practice a few times, and you'll be ready for your next quilt.