Quarter Square Triangles!
Lots of blocks use quarter-square triangles because we need the straight of the grain on the LONG edge of the triangle. Half-square triangles have the bias edge on the long side, but these are a bit different.
What is the formula to keep in mind for when you need to figure out what size of squares to cut with an X to get the 4 quarter square triangles? Take your finished size of the unit.
If the finished size is 4". Add 1-1/4" to that measurement.
In this case that size is 5 1/4"!! Cut the squares, and then cut the squares again from corner to corner in both directions with an X. This puts the straight of grain on the longest edge of the triangle. In the future, just remember that you add the 1-1/4" to the FINISHED SIZE of the unit!