Half Square Triangles; I've seen several tutorials, and several tools to buy to make them, but I found an old quilt book I'd bought in the early 90's. It had instructions how to make HST and it's my preferred method.
Lay your two fabrics right sides together and press.
Draw a grid of squares 1" larger than you want the finished HST to be.
Mine were finishing at 4" HST, so the squares were drawn at 5"
Draw diagonal lines (as shown)
These are going to be the cutting lines.
Pin between the cutting lines
Each square will make 2 HST's.
Sew 1/4" on either side of the diagonal cutting lines
Cut along all the cutting lines; the squares and then the diagonal lines.
Open and press to the darker side, and trim to the 4 1/2" square, which once sewn into the project becomes a finished 4" HST.