How to Cut Setting Triangles for On-Point Quilts

Updated June 18, 2015.

Setting and Corner Triangles Fill In the Gaps Around On-Point Quilts

Setting triangles fill in the triangular gaps that are created along the sides of a quilt when blocks are sewn together diagonally to create on point quilt layouts. More triangles are sewn to complete the corners of the quilt unless the quilter decides to use angled edges in those areas (my Medallion Star quilt is an example).

Quilters sometimes opt to fill in both areas with partial quilt blocks, like I did for a wallhanging called Pitchers for Tildy's Cabin and again for the Floating Nine Patch quilt. But it's far more common (and easier) to cut setting triangles from quilting fabrics.

One warning -- never cut patchwork setting triangles by dividing a block into halves or quarters. The triangles you produce will be too small.

How Corner and Setting Triangles Differ

Two types of triangles are used for setting and corner triangles, and although they may look alike, their structures are very different. One type has straight grain edges along its shortest sides while the straight grain flows parallel to the longest side of the other.

Why Does Fabric Grain Matter?

To minimize stretch, it's best to assemble quilt components with the fabric's straight grain along edges that will be on the outer perimeter of a block or quilt, so that there's less chance of stretch as you work.

That difference may not seem important, but makes a huge impact on the stability of the outer edges of your quilts.

How to Cut Corner Triangles

Corner triangles are half-square triangles that are created by cutting a square of fabric in half once diagonally to produce two triangles with the straight grain on their short edges.

The stretchy bias runs along a corner triangle's longest edge, and becomes stabilized when it's sewn to a quilt block.

Corner Triangle Instructions

Cut two corner triangles by dividing a parent square once diagonally. Use this formula:

Determine the size of the triangle needed by multiplying the finished block size x 1.41.
Divide the answer, the finished diagonal, by 2.
Add 0.875" and round up to the nearest 1/8" to find your parent block size.
Cut two parent blocks that size and divide each in half once diagonally to make a total of four corner squares.

The tiny bit added to round up probably won't be noticeable when you sew the triangles to the ends of rows.

Parent Squares for Common Quilt Block Sizes

4" blocks: use 3-3/4" parent squares
6" blocks: use 5-1/8" parent squares
9" blocks: use 7-1/4" parent squares
10" blocks: use 8" parent squares
12" blocks: use 9-3/8" parent squares
14" blocks: use 10-3/4" parent squares
15" blocks: use 11-1/2" parent squares

How to Cut Quilt Setting Triangles - Janet Wickell — Cut Quilt Setting Triangles. Janet Wickell

How to Cut Setting Triangles

Setting triangles are quarter-square triangles that are created by cutting a square of fabric in half twice diagonally to produce four triangles with the straight grain on their long edge -- the edge that becomes the outer perimeter of a quilt unless borders are used.

If setting triangles were cut like corner triangles, their longest edges would flow along the stretchy bias, making the outer edges of a quilt more likely to stretch out of shape before the project is complete.

Setting Triangle Instructions

Cut a parent square twice diagonally to produce four setting triangles. Use this formula to cut setting triangles:

Multiply the finished block size x 1.41.
Add 1-1/4" to the dimension calculated in Step 1 and round up to the nearest 1/8".
Create a square with sides that match the final dimension.
Cut the square in half twice diagonally. You may need to carefully rotate the fabric after making the first cut to avoid cutting backwards with a rotary cutter (an unsafe move).