Coming up in the triangle LS 10, this seam isn't quite a Y seam as there is no seam leading to the pivot point. I tend to tackle these a different way, working from the pivot out. First determine where the pivot point is.
Line up the two patches, using a pin to get the pivots together, and stitch the first side from the Pivot to the outside edge.
I have used contrasting thread so you can see my stitching, and I also do two or three backstitches when starting, to strengthen that part.

Once the first side is stitched, snip the purple fabric to within a thread or two of the pivot point. This will allow the purple to pull open so that the second side can be stitched.

To start the second side, put the machine needle back into the pivot point. You will have to pull the purple around so that it can be lined up with the white section. Some creative origami to the left, to get rid of the bulk helps, especially if you are PPing as the paper adds extra bulk.

Pull gently so that there aren't any creases towards the pivot, line up the two edges, and stitch as before, doing one or two back stitches again to strengthen the area where you snipped.

Thanks Lesley, I love the idea of starting at the pivoting point!!

Ta Dah! one completed inset seam. If you do have a little wrinkle near the middle, it can often be steamed into submission with the judicious use of an iron.

Thanks Lesley, you are a star.

Elle

Nice tute Lesley!! Thanks! Which week is this one for the DJ?

LS 10 is the triangle for week four, and has two inset seams just like this one.

RS 6 is the triangle for week five, but I recommend that you try TR 9 from week six first. They both have Y seams, but TR 9 has less of them. You can use this inset seam method for Y seams, still starting from the pivot point and working out. You just eliminate snipping to the pivot point. Instead, undo a few stitches from the central seam (the bottom part of the Y). Hope that makes sense.

Great tute Lace!!!!!

you can put the pivot point in on the y seams as well, it is where the three seams come together. the only real difference is that the shape is made from two fabrics instead of it being cut out of one. so this can also be done in one seam but some find it easier to do it in two,

Excellent!!!!! Thank you SO MUCH!