The versatile HST (half square triangle) is an amazing unit - and a block all on its own too! But add an additional bit of piecing and you have another wonderful, transformative unit - the partial half square triangle (referred to as the partial HST for the rest of the article).
Let's take a look!
Let's look at making a partial HST with the end goal of making a double pinwheel block like we see in Whirlygigs.
Partial HST Step 1
Begin with two same size squares. These will be your inner or small blades on the Double Pinwheel.
Place the squares right sides together.
On the wrong side of one of the squares, draw a line corner to corner.
Sew 1/4" to the left and right of the line.
Cut along the line and press the resulting HSTs (half square triangles).
Cut these HSTs ACROSS the seam line, corner to corner.
Note: if you want to make double pinwheels, you will need to make FOUR pairs of HSTs (ie start with four sets of squares). Look carefully at the last step - the cut partial HSTs are mirror images, not identical (that's why you need to make twice as many!). Good news - you can use the other units to make a second block that "spins" the other way!
Partial HST Step 2
Cut a larger square in half corner to corner.
This larger HST will become the larger blade in your partial HST unit.
Partial HST Step 3
Match up the long cut edge of the large HST and the partial HST. Sew with a 1/4" seam allowance.
If you want to take it a step further and construct a double pinwheel block, you will need to make four IDENTICAL sets of partial HST units.
When making a double pinwheel, lay out your units to determine proper placement of each unit. Then stitch together as you would any pinwheel block. Easy, right?!
Is this the ONLY way to construct a partial HST? Like most units and blocks, there are usually slight variations and versions.
Head over to Kate's blog to get some additional ideas on the partial half square triangle as well.
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