Getting a nice square half-square triangle unit can be frustrating. Many folks will make them over-sized and trim off the wonky edges.
But we didn’t cut them wonky, so why end up with not-square units?
It’s all about that bias edge. Depending on how you make your units, there’s always a bias edge. Bias refers to the direction that runs diagonal to the straight of grain, also known as the lengthwise and crosswise grain.
Bias edges stretch. Handle that bias cut edge or edges carefully to avoid stretching it. Pressing with Best Press or similar product before cutting can help.
How the diagonal seam is pressed makes a big difference too. See the diagram below.
Instead of running the iron along the seam, press in a two-part motion first across the block with the grain running crosswise, then up with the lengthwise grain. This is still pressing, not ironing! I guarantee this will keep your HSTs squarer.