More often than not, it's smallest details that make the biggest difference to the finish of your handmakes. But knowing what technique to apply, and when, and why, can sometimes baffle even a seasoned seamstress. If sewing a curved seam was already tricky enough, then getting it to sit perfectly crisp once it's pressed can prove to be even more frustrating... Unless you know when to clip and when to notch.
Clipping your seam allowance
If you have a concave seam - one that looks like a smile - you'll need to clip the seam allowance. This type of seam would most likely be found on a neckline or a curved side slant pocket.
Once you've sewn your concave seam, have a go at turning it to the right side and getting it to lie flat. It won't. The shorter curve that is the raw edge of your seam allowance will stop you from being able to spread the seam allowance out in order to press.
By simply snipping, or clipping, into the seam allowance getting nice and close to your line of stitching - but not too close! - you will release the tension and find that pressing your seam out to the right side is now easy as pie.
Notching your seam allowance
No points for guessing that if we clipped a concave seam, then surely notching must be applied to a convex seam, ie. a seam looking a little like a hill or the top of a bowler hat. You'll find these sorts of seams on bust shaping princess seams and scalloped hems.
Again, if you try to turn your convex seam out to the right side pre-notching, you'll find that the excess seam allowance bunches up as the curve of the raw edge is longer than that of the seam. Notching helps us to get rid of this unwanted excess.
To notch your convex seam, snip little triangles out of the seam allowance every 1/2" or so. Get nice and close to your stitches, without snipping right through them of course.
Turn your seam out to the right side, give it a good press and revel in that lovely un-bunchy flatness!