Quilts with blocks set on point have triangular openings around their outer edges where each diagonal row of blocks ends. You can assemble triangular half-blocks to fill in the spaces, but quilters often use large fabric triangles instead.
Two types and sizes of triangles are used to fill in the blanks. One size fits into the gaps along the top, bottom and sides of the quilt. Four smaller triangles are used to create the quilt's corners.
Rotary Cut Half Square Triangles (setting triangles)
Cut a parent square in half once diagonally to produce corner triangles for on point quilts. Rotary Cut Quarter-Square Triangles (corner triangles)
Cut a parent square in half twice diagonally to produce four setting triangles for on point quilts
To minimize stretch, it's best to assemble quilt units with the fabric's straight grain along edges that will be on the outer perimeter of a block or entire quilt, so that there's less chance of stretch as you work. If setting triangles were cut like corner triangles, their long edges would be cut on the stretchy bias, making the outer edges of a quilt stretchy — those edges sometimes tend to stretch out of shape a bit anyway as you finish the project.
Cut two corner triangles by dividing a parent square once diagonally.
Start by calculating the finished length of a block's diagonal.
Finished block size x 1.41 = finished diagonal.
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.
We'll cut a parent square twice diagonally to produce four setting triangles with the fabric's straight grain on their long edges. Use this formula for setting triangles:
Finished block size x 1.41 = finished length required on the triangle's longest edge. Round up to nearest 1/8".
Cut a square with sides that are 1-1/4" longer than the number in Step 1.
Cut the square in half twice diagonally.