Before there can be pinwheels

There have to be half square triangles.  Or half square triangle blocks.  Or half square triangle squares.
Whatever you choose to call them, you'll have to make them if you want to make pinwheels.

So, how do you make a half square triangle (or two)?

By sewing together two fabric squares!

What size should the squares be?
Follow the golden rule:  each square should be the size of your finished half square triangle + 7/8".

So...
If you want to make a 4" finished half square triangle block (well, you'll end up with two), you have to cut two 4 7/8" x 4 7/8" squares, one in each color you want in your finished block.

I've decided that, if I'm going to be cutting up fabric, I'm going to make something. If you want to follow along on this post and the next, you will have an 8" x 8" pinwheel, which I'm going to make into a potholder.

Here we go:

1. Cut two squares the size of your finished half square triangle + 7/8" each. (Mine are 4 7/8" x 4 7/8")
   
2. On the back of the lightest square (or the one with the lightest wrong side), draw a diagonal line corner to corner.  You can use a permanent marker or pencil for this because it's not going to be in the block itself, but if you're using light colored fabrics, I'd recommend a washable or disappearing marker.
   
3. Place the square you drew the line on right sides together with the other square you cut.
4. Stitch a seam 1/4" away from the diagonal line.  Stitch another seam 1/4" away from the diagonal line on the other side.
   
5. Cut the two sections apart along the diagonal line.
   
6. Press the blocks open with the seam allowance toward the darker fabric.
7. Voila!  You have two half square triangles!
   

You will need four half square triangles for each pinwheel, so you'll need to make two more like this.

Tomorrow we'll make the pinwheel and I'll finish my potholder.