Two squares go in. Four half square triangles come out.
That is the appeal, and it is a real one when a quilt wants dozens of them. But this method leaves something behind that decides where those units are allowed to sit in your quilt, and it is the reason Yvonne teaches it in the same breath as the two at a time version. Miss it and your block grows. Here she is with the whole thing, from her studio.
The method takes about a minute to learn. Everything after that is knowing when to reach for it.
Take It to the Machine
Two sheets, both free to print, use and teach. The chart above is on the half square triangle size chart, with the three methods side by side. The whole of this tutorial, the method, the sum and the chart, is on the four at a time guide. Nothing on either one needs to print at actual size.
Two Squares, One Trip Around the Outside
Put two identical squares right sides together. Sew a quarter inch from the edge all the way around, all four sides.
Yvonne stops a quarter inch short of each corner and pivots, but that is a preference rather than a rule. You can sew off the edge, cut, and start the next side. She pivots because her machine's cutting function takes a moment and pivoting is quicker.
Then comes the cut that people get wrong. You cut corner to corner, twice, but not from the fabric corner. From the stitching corner. The two are not always in the same place, and it is the stitching that has to line up.
One small thing that will save you squinting: use a contrasting thread. Yvonne sewed hers in white on white and says so herself, that she has to learn to use a thread you can actually see.
Open the four pieces out and you have four half square triangles from one trip around a square.
Where the Bias Ends Up, and Why You Should Care
Now the part that matters.
Take one of the units and pull gently on the seam side. Nothing. That edge is on the straight of grain and it does not move. Now pull on either of the outside edges. They give.
That is the difference between this method and the two at a time version. Two at a time puts the bias on the inside of the unit. Four at a time puts it on the two outer edges, where every quilter handles it most.
None of that makes the method worse. It makes it specific.
Where These Units Belong
A stretchy outer edge on the corner of a quilt block is asking for trouble. It pulls while you sew, your seams stop matching, and the block will not sit square.
Put the same unit in the middle of a block and the picture changes completely. Three of its sides get sewn onto straight of grain fabric, and those seams hold it steady. Only one span is left with any give in it at all.
Yvonne shows this with two blocks from the week before, made from identical pieces in different layouts, one of them a shoofly. On one, the half square triangle lands on the outside corner, and there she wants straight of grain, no argument. On the other, the same unit sits in the center, and a little bias there is fine. She says it can even help, because you can nudge things into place.
Her size limit is worth writing down. At 4 1/2 or 5 inches she is happy to use these in the middle of a block. Much past 5 or 6 inches and she will not, because that is too long a span of bias to trust.
Pressing, Then Trimming
Set the seam first. Hit it closed with the iron before you open anything, which locks the stitches so they do not pull. Then roll the unit over on itself and press toward the darker fabric.
The units will come off the machine slightly oversized, and that is deliberate. Yvonne cuts her squares a little large so there is something to trim.
To trim, find the 45 degree line on a square ruler and lay it along the diagonal seam of the unit. Check that the size you want falls inside the fabric on both sides. Cut those two sides, turn the unit, and cut the other two.
One habit worth stealing, and it costs nothing: cut standing up. You get downward pressure through the blade and noticeably better accuracy.
The Only Sum in This Tutorial
Start with the finished size you want each unit to be. Add the half inch of seam allowance back on. Divide that number by .64, add a quarter inch of wiggle room, and round to a quarter inch. The chart above has the usual sizes already worked out.
So for a unit finishing at 4 inches: 4 plus 1/2 is 4 1/2, divided by .64 is 7.03, plus the quarter inch is 7.28, which rounds to 7 1/4. Cut two squares at 7 1/4 inches, sew, cut, and trim the four units down to 4 1/2.
You can run it backward too, which is what you want when you are starting from a precut rather than from a pattern. Take the quarter inch of wiggle room back off the size of your square, then multiply by .64. A 5 inch charm square lands just over 3 inches, which trims comfortably to a 3 inch unit.
Two, Four or Eight
Three methods, and one question decides between them, all three worked through in the full guide to half square triangles: how many identical units do you need, and where in the block are they going?
Two at a time when you need a pair, and whenever the unit is heading for the outer edge of a block, because it leaves the straight of grain where you want it.
Four at a time when you need exactly four, and they are going somewhere the neighboring seams will hold them steady.
Eight at a time, the Magic Eight, when a block wants six or more of the same unit and one pass can hand you the whole set.
Cut one test pair before you cut forty. Sew it, press it, trim it, measure it. Four minutes now against an evening with the seam ripper later is not a difficult trade.
Which is your go-to method for half square triangles: two at a time, four, or something else entirely? Tell us in the comments below. We always love hearing how you work. And if you have not already, subscribing to our channel means you will catch every new tutorial from Yvonne as soon as it lands.
