A QuiltPlan piecing guide
Half-Square Triangles
Two at a time, trimmed true.
A square split corner to corner into two triangles: the unit behind pinwheels, stars, flying geese corners and a great many traditional blocks. This is the classic way to make them, two at a time from a pair of squares. The squares are cut a little big, so each unit can be trimmed square with its seam running exactly corner to corner, which is what makes the points meet.
What you cut, for two units
Shown for a unit that finishes at 3″: one light and one dark square, each 4″. Every size is in the chart below.
| Finished unit | Cut 1 light + 1 dark square | Trim each unit to | Cut exact, no trim |
|---|---|---|---|
| 1″ | 2″ | 1½″ | 1⅞″ |
| 1½″ | 2½″ | 2″ | 2⅜″ |
| 2″ | 3″ | 2½″ | 2⅞″ |
| 2½″ | 3½″ | 3″ | 3⅜″ |
| 3″ | 4″ | 3½″ | 3⅞″ |
| 3½″ | 4½″ | 4″ | 4⅜″ |
| 4″ | 5″ | 4½″ | 4⅞″ |
| 4½″ | 5½″ | 5″ | 5⅜″ |
| 5″ | 6″ | 5½″ | 5⅞″ |
| 6″ | 7″ | 6½″ | 6⅞″ |
One pair of squares makes two units. The squares are cut 1″ bigger than the finished size, which leaves a little over 1/16″ on each side to trim true. The last column is the exact size: it leaves almost nothing to trim, so it only works if every seam is exactly a quarter inch.
Before it is sewn into anything
A 3″ half-square triangle measures 3½″ square before it is sewn into a block, and its seam runs exactly into two opposite corners. The quarter inch seam line all round crosses that seam at its own corners too, which is why a seam that hits the corners gives a point that meets.
Making them
1 · Mark the light square
On the wrong side of the lighter square, draw a line from corner to corner with a sharp pencil or a fabric marker. A line on the light fabric is easy to see, and it will not show through.
2 · Sew either side of the line
Lay the light square on the dark one, right sides together, edges matched. Sew a quarter inch from the line on one side, then a quarter inch from it on the other. Nothing has been cut on the bias yet, so nothing stretches.
3 · Cut on the line and press
Cut along the drawn line. You have two units. Open each one and press toward the dark, pressing down rather than sliding the iron, so the bias seam does not stretch. The seam allowance pokes out past two corners: those are the dog ears, and the trim takes them off.
4 · Trim it to 3½″
Use a square ruler with a 45° line. Lay the 45° line exactly on the seam, with the ruler's 3½″ lines inside the unit and a little fabric past its right edge and its top. Trim the right and the top. Turn the unit around, lay the 45° line on the seam again and line the two cut edges up with the 3½″ lines, then trim the other two sides.
5 · Check it
It measures 3½″ each way and the seam runs exactly into two opposite corners. If the seam misses a corner, the point will float or be cut off when the unit is sewn in: trim the next one with the 45° line placed more carefully on the seam.
6 · Sew it in
Sewn to the next piece with a quarter inch seam, the triangle's seam runs right into that seam, and the next seam across will cross it at the same spot: a sharp point. Sew with the triangle unit on top, so you can see where the seams cross.
Good to know
- Why 1″ bigger? Sewn and pressed, a unit comes out a little over ⅜″ smaller than the squares it was cut from (the quarter inch seam, running on the diagonal): 4″ squares make a unit a little over 3⅝″. Cut 1″ over, there is a little to trim on every side; cut exact (⅞″ over), there is almost nothing, and a seam a thread too wide makes the unit short.
- Pressing. Toward the dark keeps a dark seam allowance from showing through the light fabric, and it lets the seams nest when units are sewn together. For very small units, some quilters press the seam open to cut the bulk where points meet.
- Press before you trim. The 45° line needs a pressed, flat seam to sit on.
Designed to print. Use your browser's Print (Ctrl/Cmd+P) for a clean letter-size page.