The angles used in the Tri-Recs tools are:

90, and approximately 63.435, and 26.565 degrees for the skinny triangle,
and

approximately 63.435, 63.435, and 53.130 degrees for the "center' triangle.

If one does not have one of the Tri-Recs types of rulers, it is quite easy to draft one's own template for the unit.
Post #13 gives an example of what one would look like.

Gay: post #15, has the correct way to sew two rectangles together to get a half rectangle triangle. Is that an HRT?

If you cut the rectangle in half on the bias, that will work also, but then you are sewing on the bias.

RE: post #15 diagram

Just make sure your A and B rectangles are oversized enough to get the correct size on your A/B HRT after you cut on that center line and 'rectangle up' the resulting HRT. A half inch isn't enough.

This will not work "I may be wrong but I would take 2 rectangles 4 1/2x 3 1/2 then sew and cut as you would a HST. Anxious to see how wrong I might be but that's how I would do it."

I remember struggling with this a while back when doing a BOM. If memory serves me correctly, I finally paper pieced the rectangles to get them to turn out the right way (and the right size). Good luck with your effort!

Linda in Missouri