Techniques · Piecing
How to sew Y-seams (set-in seams)
Where three pieces meet at a point, no seam can run straight across. Here’s the one rule that makes set-in seams lie flat, and a Le Moyne Star sewn step by step.
The short answer
Mark a dot ¼″ in from each corner, and sew each seam only from dot to dot, backstitching, never into the seam allowance. Sew the first side of a set-in piece to the dot, stop with the needle down, swing the piece round to the next side, and sew out from the dot.
What a Y-seam is
Most piecing is straight seams sewn edge to edge. A Y-seam, or set-in seam, is where three pieces meet at a point and no seam can run straight across: the three seams make a Y. You can’t sew one long seam, so you sew each one only as far as the point, and stop.
The Le Moyne Star is the classic: eight diamonds meet in the middle, and the background squares and triangles are set into the gaps between their points. Each ring below is a Y-seam.
The one rule: stop at the dot
Every seam of a set-in piece is sewn from dot to dot, the points ¼″ in from each corner where the sewing lines cross. You never sew into the seam allowance at a Y. That free allowance is what lets the next piece swing into place and lie flat.
So mark the dots. Templates for set-in pieces show them: push a pin through each dot on the template and mark it on the wrong side of the fabric. Here are the Le Moyne Star’s three templates at 12″, each with its dots:
| Template | Cut from | For one block |
|---|---|---|
| A Diamond | 3″ strips: 9 a strip | 8: 4 Dark · 4 Medium |
| B Corner square | 4⅛″ strips: 10 a strip | 4 Background |
| C Side triangle | 3⅛″ strips, turned each cut | 4 Background |
Cut the diamonds with the template or a ruler’s 45° line, so the strip’s straight grain runs along two of each diamond’s sides. The corner squares and side triangles go on the outside of the block, so cut them with their outer edges on the straight grain.
Sewing a Le Moyne Star
Mark the dots on the wrong side of every piece, ¼″ in from each corner.
Sew the diamonds in pairs. Pin two diamonds right sides together with their dots matched. Sew from the outer dot to the inner dot only, backstitching at both. Make four pairs.
Sew the pairs into halves, and the halves into the star, the same way: dot to dot, backstitching, never into the allowance. Where eight points meet, push a pin straight through the dots to line them up.
Set in a corner square (1). Pin one side of the square to one diamond, dots matched. Sew from the outer edge in to the dot and stop, needle down. Lift the foot, swing the square’s next side round to the next diamond, pin, and sew from the dot out to the edge.
Set in the side triangles (2) the same way, into the four gaps that are left.
Press. Press the seams around the center all one way, so they spin and lie flat. The block measures 12½″ (12″ finished).
Common mistakes
- A pucker at the Y. A stitch went past the dot into the allowance. Unpick that one stitch and it lies flat.
- A hole at the Y. The seam stopped short of the dot. Sew the last stitch or two to it.
- Stretched diamonds. Two sides of a diamond are on the bias. Handle them lightly, and press, don’t iron back and forth.
- A star that won’t lie flat in the middle. Usually the diamonds were cut a little off 45°. Check the template against the ruler’s 45° line.
In Quarter Inch: a block with set-in seams says so on its cutting chart, prints its templates with the dots marked, and shows which pieces to set in first. See Templates and How to sew.
Questions
Short answers.
Something else? A real person replies.
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What is a Y-seam in quilting?
A place where three pieces meet at a point and no seam runs straight across, so the seams make a Y. Each seam is sewn only up to the point.
Why does my Y-seam pucker?
A stitch went past the dot into the seam allowance. Unpick that stitch, so the allowance can swing free, and it will lie flat.
Do I need to backstitch on a set-in seam?
Yes, a stitch or two at each dot. The seam ends inside the block, so nothing else holds it.
Can I sew Y-seams by machine?
Yes. Sew to the dot, stop with the needle down, lift the foot, pivot the next piece into place and carry on. Many people find them easier by hand at first.