The quarter-square triangle formula
A quarter-square triangle (QST), also called an hourglass unit, is a square split along both diagonals into four triangles. Because there are two diagonal seams, it needs more extra than a half-square triangle:
cut square = finished size + 1¼"
If you prefer to trim your units, add 1½" instead and trim to finished size + ½".
How to make two QSTs at once
- Cut one light and one dark square at the size above.
- Make two half-square triangles: draw a diagonal, sew ¼" on both sides, cut on the line and press.
- Place the two HSTs right sides together with opposite colors facing, nesting the seams.
- Draw a diagonal line across the seam, sew ¼" on both sides and cut on the line. You now have two hourglass units.
Example
For 4" finished hourglass units, cut 5¼" squares. Each light/dark pair makes two QSTs that measure 4½" unfinished.
Frequently asked questions
Why add 1¼" for quarter-square triangles?
The unit has two diagonal seams plus the outer seam allowances. The standard allowance that accounts for all of them is 1¼" over the finished size.
How many QSTs does one pair of squares make?
Using the two-HST method, one light and one dark square make two quarter-square triangle units.
What is the difference between a QST and an HST?
An HST is split once on the diagonal into two triangles; a QST is split on both diagonals into four. QSTs keep the straight grain on the outer edges of the block.