Half Square Triangles

Learn how to make half square triangle blocks as well as learning where to use half square triangles.

What are Half Square Triangles?

A Half Square Triangle is a square that is cut through the diagonal to give two triangles. Two half square triangles make a square.

half square triangle

half square triangle

How are they made?

HST lesson seam drawingCut two squares of fabric the required size. For this lesson, we will use 3" squares. 

On the lighter fabric, draw a line from upper corner to lower corner through the center point of the square. 

Place both squares, right sides together, and stitch ¼" from drawn line on both sides. Cut along line.

Press seam as it is. This sets the stitches and keeps the block in shape. 

Place block, withdarkest square on top, on ironing board. Lift top triangle and press over seam.

HST lesson pressing

pressing triangles

Using a square ruler, match diagonal line with seam line and trim block to a 2½" square.

HST lesson block trimming1 HST lesson block trimming2

One set of squares makes two half square triangle blocks.

Why should we not cut the squares before we stitch them?

Stitching before cutting is a much more accurate way of making half square triangles. Since we are stitching on the bias of the fabric, the seam can stretch easily. By stitching through the square, there is less chance of the seam stretching out of shape, making a much more
accurate block.

How do you determine the cutting size?

Add 1" to the finished size of the block. This example shows an 8" block. The dotted lines indicate 2" grid. Half square triangles (outlined in red) for this block are 2" therefore cut each square 3".HST lesson block corner triangle

HST lesson quilt corner triangleWhere else do we use half square triangle?

Half square triangles can also be used for corner setting triangles of a quilt made where blocks are on-point (outlined in the red triangle). Here
we just use single triangles so they are cut before stitching.

Do we also use single half square triangles for the setting triangles on the side of a quilt made with blocks on-point?

No. We use quarter square triangles for this (quarter square triangle is outlined in green).

Why do we use half square triangles for the corner setting triangles and quarter square triangles for the sides of an on-point quilt?

This is so when we add borders or binding, we are stitching along the grain of the fabric rather than the bias.

Why is it necessary to apply borders on the straight of grain instead of the bias?

This reduces the likelihood of the quilt top stretching out of shape while stitching. If the quilt top stretches, it can create wavy borders on your quilt.

Check out this pattern that uses half square triangles

a freshness of spring

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Note: these instructions are copyrighted. It is illegal to copy them, distribute or sell them without permission. Thank you for respecting the rights of the designer. Please contact Arbee Designs for permission if you intend using this page for anything other than your own use.

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