On-Point Quilt & Setting Triangles

On point is exactly twice the quilt

A block set on point stands on its corner, so it occupies its diagonal rather than its side — side × √2. A 12-inch block takes up 16.97 inches once it is turned.

That is 1.414 times bigger in each direction, and since √2 squared is 2, an on-point quilt with the same number of blocks is exactly twice the area. Not roughly — exactly, at every block size and every layout. People turn a planned quilt on point "to make it a bit bigger" and double it, then wonder why they have run out of everything.

There is a second surprise in the block count. An on-point setting has the main grid plus a block in each gap between them, so a 4×5 setting is not 20 blocks — it is 20 + 12 = 32. Both numbers move against you at once.

Side and corner triangles are not interchangeable

This is the classic on-point mistake, and it does not show until the quilt has a rippling outer edge that cannot be quilted out. The two kinds of triangle are cut differently because of where the straight grain has to land:

• Side triangles are quarter-square triangles. Cut a square on both diagonals. That puts the straight grain on the long edge, which becomes the outside of the quilt. Two bias seams, so the 1.25″ allowance.
• Corner triangles are half-square triangles. Cut a square on one diagonal. That puts the straight grain on the two short legs, which become the quilt's corners. One bias seam, so the 0.875″ allowance.

Swap them and you put bias on the unsupported outer edge of the quilt, which is the one place fabric has nothing to hold it. It stretches while you handle it, and no amount of careful quilting will pull it back.

The cutting sizes are not new rules

Worth knowing, because it means you only have to remember two numbers for the whole craft. The setting-triangle allowances are the same two allowances as half-square triangles and flying geese — one bias seam costs 0.875″, two cost 1.25″ — picked here by which grain has to face outward rather than by anything new.

Side squares are cut into four triangles each, corner squares into two. All of these come out generous on purpose — trim the setting triangles flush with the quilt top once the rows are joined, which is much easier than trying to cut them exactly.

Notes

• Blocks across and down are the main grid. The alternate blocks that fill the gaps are counted for you — you do not enter them.
• Assemble in diagonal rows. An on-point quilt is not sewn in horizontal rows; it goes together in diagonals, each row starting and ending with a setting triangle. Lay it out before you start sewing, and label the rows.
• Handle the bias gently anyway. Even with the grain the right way round, every setting triangle has a bias edge somewhere. Starch helps, and so does not letting the quilt top hang off the edge of the table.
• Borders go on afterwards. The size shown is the quilt top before any border, which is the number a border calculation starts from.