Half Square Triangles

Posted by Ruth on 24 January 2011 | 0 Comments

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What are Half Square Triangles?

half square trianglesA Half Square Triangle block is a square that is seamed through the diagonal from corner to corner. It is made of two half square triangles. Each triangle is a different fabric. 

How are they made?

draw line through lightest squareCut two squares of fabric the required size. For this lesson we will use 3" squares to make a 2" finished block.

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

Stitch both sides of linePlace both squares, right sides together, and stitch a scant ¼" from drawn line on both sides.

Cut along line.Cut along solid line


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

2: Place block, with darkest fabric on top, on ironing board. Lift top triangle and press over seam.

Press seam flat   Use tip of iron to press top fabric over seam  Use tip of iron to press top fabric over seam

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

trim to 2 1/2"  trim to 2 1/2"

One set of squares makes two half square triangle blocks.

More Questions about Half Square Triangles

Why should we not cut the squares in half 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. We can ensure our center seam can align with the ruler for trimming.

How do you determine the cutting size?

Add 1" to the finished size of the block. Here is an example showing an 9" block. The dotted lines indicate a 3" grid. Half square triangles (outlined in red) for this block are 3" finished therefore cut each square 4".nine-patch with half square triangles

More examples:

For a 2" finished square you would cut 3" squares.

For a 2 1/2" finished square you would cut 3 1/2" squares.

For a 1 3/4" finished square you would cut 2 3/4" squares.

Where else do we use half square triangle?

corner setting trianglesHalf 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?

side setting trianglesNo. 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.

We hoped you enjoyed learning all about half square triangles

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