I want to talk about when to use a half-square triangle, and when to use a quarter-square triangle. I have actually seen this written wrong in patterns before, so hopefully this will help you if you run into a pattern that is done incorrectly.
This is a half-square triangle. It is cut once across the diagonal.
This is a quarter-square triangle. It is cut twice across both diagonals.
In deciding which one to use, it is all about the grain. A half-square triangle has the straight grain on both short sides, and the bias on the long side. A quarter-square triangle has the straight grain on the long side, and the bias on both short sides. Whether making a single block or finishing a quilt, you want the straight grain on the outside.
For instance, in this block, you would use 4 half-square triangles, cut from 2 squares. This puts the straight grain on the outside of the block.
In this block, you would use 2 quarter-square triangles (for the brown fabric). Again, this puts the straight grain on the outside of the block. (The colored pieces would be half-square triangles.
Are you beginning to see how it works? You often see both of these kinds of triangles when setting a quilt on-point. You need both kinds of triangles when doing the setting. The half-squares triangles are on each corner of the quilt, and the quarter-square triangles are on the sides of the quilt. Now you have a straight grain all around the quilt top.
I posted a setting and corner triangle chart that is helpful in setting quilts on point, and also a chart of the diagonal sizes of quilt blocks. And, just for funsies, here’s an on-point quilt of mine, Pastelmania.
I hope you find this post useful!