Diagonal Seams Half Square Triangle


Diagonal Seam Half Square Triangle

This method starts with two slightly oversized fabric squares and creates two half square triangle (HST) blocks at a time – perfect for those times when you want accurate blocks and pointy points

Step 1

Find the finished size of the HST patch. Add 1-inch to that dimension and cut two fabric squares that size.

Cut size = Finished size + 1-inch

Example: For finished size = 2 ½

Cut size = Finished size + 1-inch = 2 ½ + 1 = 3 ½. Cut 2.

Step 2

Layer fabrics right sides together. Draw a diagonal line from corner to corner.


Step 3

Sew a seam ¼-inch away from both sides of the drawn line. Cut on the drawn line.


Step 4

Open block and press. You will have two blocks.


Step 5

Line up the 45 angle line of your ruler on the diagonal seam and make sure the unfinished size of the patch is on the fabric (3-inch x 3-inch in our 2 ½-inch finished square example). Trim to size.



  • Sew parallel to the drawn line from beginning to end of the seam
  • Larger square ruler provides more stability during trimming. Grip dots on the ruler are also useful
  • Bloc_Loc makes specialty rulers with a seam allowance groove for trimming HSTs. It is a uni-tasker tool which works really well