Y-seam construction is used when 3 seams meet in 1 place in the quilt block, forming a “Y”. The example below includes 2 diamond shapes and a triangle which is sewn to each diamond.
1. Mark small dots at the seam line corners on all 3 pieces to be joined.
2. Pin and stitch the first seam, stopping your stitches just before entering the dot, and then backstitching. Stitch up to the dot, as close to it as possible. (Make your stitch length smaller, if necessary.) Open the pieces. Finger press the seam allowance away from the seam of the next piece to be added.
3. To join the third piece, line up the corner dots and stitch as before, backstitching before entering the dot. Do not stitch into the previous seam allowance. Open the pieces.
4. Stitch the remaining seam, ending just short of the dot at the “Y”. Open and press.
This method is described in Mastering Precision Piecing, by Sally Collins.