How to sew the perfect Y-seam

What’s a Y-seam? When would you need it?

Y-seams are also referred to as inset seams or setting in
Y-seams are seams where three pieces of fabric meet. You can’t  sew pieces together using standard straight-seam sewing. 
The thought of sewing them can be daunting, but it doesn’t have to be like that.
I’m well overdue with getting to grips with Y seams.
I’ve sewn them in the past but never been really happy with how they turn out. I could never get a crisp, flat, accurate join between the three pieces.
But when I designed my Tudor Rose quilt block I decided that I had to master the Y seam once and for all. 
And guess what? It’s actually easy to get those three angles to meet perfectly.

Can you see the two blue triangles in each corner?
Those are sewn in using the Y-seam method.


When it’s sewn accurately the block should lie flat with no bubbling or puckering. 
Don’t be afraid of the Y-seam! 
  • Accuracy of cutting is crucial. Y seams usually involve stitching on the bias, so it’s important to stabilise your pieces before you sew.
  • Starching and pressing helps, and try to handle the pieces as little as possible.
  • If you like, mark the intersection for perfect alignment. A dot on each of the three pieces shows exactly where the stitching ends. (But see my quick 
  • Stitching always starts/ends 1/4″ from the raw edge.
  • Secure the end with a couple of back stitches
Sew along with me to make a tumbling block. Once you’ve mastered the technique you’ll be able to tackle Y seams in any quilt.
This is a long post with lots of details but don’t let that put you off.
Once you’ve made a couple you’ll be whizzing up Y-seam blocks in no time.

Example: Tumbling Block

What you will need 
  • scraps of three different fabrics: light, medium and dark.
  • Starch and a hot iron
  • 1/4″ foot, or a standard foot with 1/4″ clearly marked. 
  • Good light!
Stitch length
My machine has a 1/4″ seam setting which adjusts the needle position and the stitch length automatically.
If you’re setting your machine manually choose a short stitch length – approx 1.8 to 2.
A shorter stitch makes it easier to hit the correct spot as you approach the corner without overshooting. Better to stop half a stitch short than sew into the seam allowance.
A short stitch length also means there’s less chance of seams unravelling as you manipulate the diamonds.

Cutting diamonds

To make a true tumbling block you must start with 60 degree diamonds with all 4 sides equal length.
The sides can be any length you like, but I suggest starting with nice large diamonds to make the pieces easy to handle.
I’m using 4″ diamonds for this tutorial.
Begin by cutting 3 3/4″ strips from pressed, starched fabric.
Take a ruler with 30˚ marked and line the 30˚ against the long cut edge. 
Trim off the top at an angle
Measure 4 1/2″ down and cut again at the same angle.
Repeat for as many diamonds as you need.
For the maths nerds amongst you, here’s a neat formula for calculating the how long to cut your strip of fabric:
total length = ((length of side + 1/2″ ) x number of diamonds) + 2 1/2″ 
to cut 6 diamonds with a finished side length of 4″ 
4 1/2″ x 6 = 27″
add 2 1/2″ = 29 1/2″
You’ll need a strip minimum 29 1/2″ long.
I’d suggest cutting the strip longer than this in case of errors: you can always trim down.

Knowing when to stop

One technique that’s crucial for sewing set in seams successfully is stopping your stitches exactly 1/4″ before the end of the seam.
Wait, how do you know when to stop?
One way is to mark the intersection of the seams on the WS of each piece of fabric. Then stop sewing when you reach the mark. 
It’s accurate, but time consuming.
If you will be sewing a lot of Y seams, marking each intersection can take forever.
Some diamond templates have a hole ready made at the intersection of the seams which makes accurate marking a lot easier, but it still means faffing about with a pen.
Here’s a quick and accurate way of getting an accurate seam end.
This method doesn’t involve any marking. You’re just going to dive in and start sewing.
You’ll need to use the 1/4″ presser foot for your machine, if you have one.
If not, a standard presser foot works just as well, (a clear one is a bonus) but you’ll need to set up a 1/4″ guide on your machine.
Find markings on your throat plate that correspond to 1/4″ so you can guide the raw edge as you sew. You can use a piece of tape 1’4″ away from the needle. Or maybe you have a groove on the presser foot that’s exactly 1/4″ from the needle. Anything that will align with your raw edges to keep the seam straight and even.
Sewing the seam
Start to sew from the narrow point of the diamond. Slow down as you approach the corner of the broad point. Lower your needle, raise the presser foot and pivot the fabric as if you were turning the corner.
If the raw edge meets your 1/4″ guide you’re in the right place.
If the edge projects beyond the 1/4″ guide, you need to take a couple more stitches.
Pivot back and sew a couple more stitches.
When you’re in the right spot, pivot the fabric back again and make a couple of reverse stitches to lock the seam.
Tip: get to know what your machine does when you reverse.
Mine makes one stitch in the last hole it sewed, then goes back along the seam. So I know I can stop right on the point and go back.
Check yours doesn’t creep forward a touch – it shouldn’t, but every machine is different.


Sewing the perfect tumbling block

Start with three diamonds, one pink, one green, one blue.

Lay the pink and blue diamonds RS together, matching all sides exactly.

Starting from the narrow point, sew towards the broad point. Use whatever method you like to end 1’4″ from the end. Make a couple of back stitches to secure the seam.

Check you have enough free seam allowance for the next step – you should have 1/4″ free along the next side of the triangle, right into the broad point.

Press the seam towards the pink diamond. 

Take the green diamond and match to the pink diamond, right sides together. If you cut your diamonds accurately it should line up perfectly. Fold up the blue diamond – you should see the corner peeking out. These corners should match exactly.

Starting from the narrow point, sew the pink and green diamonds together exactly as before. Slow down as you approach the end and check the corners are still aligned – sometimes fabric shifts as you sew. If it’s still in the right place, carry on. If not, you’ll need to unpick and start again.

TIP: starch well. Some of your seams are going to involve sewing on the bias, and starch helps keep everything stable.
As you get to the corner fold the blue diamond out of the way. This time you can use the end of the previous seam as a guide. Stop and back stitch as before.
Keep the blue diamond free. Don’t stitch into it at all. You should still be able to fold the blue diamond away from the seam you just stitched.

Press the seam towards the green diamond

Fold the pink diamond in half so you can match the blue and green raw edges.

Exactly as before, sew from the pointy angle towards the broad angle, keeping the pink diamond out of the way and checking your points match perfectly as you approach the end.
Any shifting on this last seam will result in a bubble or pucker in the middle of your tumbling square.
If you find it tricky to get the corner to match up, try sewing the other way, starting at the wide angle and sewing out to the narrow end. Any excess fabric will be pushed put to the edge where it can be trimmed away.
Press the last seam towards the blue diamond
All the little tails in the centre will naturally fall flat and form a little star shape.
You can now press the finished tumbling block.
There you have it! A perfect Y-seam
Whenever you find a seam like this, follow these steps and you’ll get flat, perfectly matched three-point corners every time.
Sometimes you can’t press the seams to fall the same way, as we did in the example. It depends where your inset seam lies in relation to other patches in a quilt block.
Don’t worry too much about it. Let the patches decide,; the seam allowances will naturally orient themselves the best way.

In summary: tips for perfect inset seams

  • starch well
  • cut accurately
  • mark the intersection or use the pivot method
  • check for fabric shifting as you sew
  • if you get bubbling, stitch the final seam from the centre of the block to the edge
  • press carefully as you go
  • larger pieces are easier to piece accurately.
  • don’t be afraid to unpick.
Finally, if you’re sewing a seam that’s inset at both ends, mark or use the pivot to find the starting point and begin the seam with a couple of back stitches. 
Why not have a go at sewing a tumbling block and share in our Facebook Group?