

For a Half Square Triangle - determine the finished size of the unit. My unit above (see the corners) finish at 4 inches. Therefore, I added 7/8 to that measurement and cut one light square and one med. blue square each at 4 7/8. Pair them right sides together, and draw a line from corner to corner. Sew 1/4 inch on each side of that line, cut them apart, press open gently. and voila, you have 2 half square triangles, aka HSTs.
For a 3-color Quarter Square Triangle - Do you see the 4, 4-piece units on the around the blue/red star, surrounding the red center? Determine the finished size of the unit. In this case it is 4 inches. ADD 1 and 1/4 inches (Quarter Square Triangle = add 1 and 1/4 inches) to a dark square and a medium square, and a second dark square and a light square (the two darks are the points of the star, in this case). So each of these 4 squares are cut 5 1/4 inches. Place right sides together of dark & medium, and the right sides together of dark & light, draw line on diagonals, and sew 1/4 inch on each side. Cut on the line, press open gently - you should have 2 HSTs, with different colorations. Now, put them together again, right sides perfectly together, and draw another line on the diagonal. Peek to be sure you have them in the right direction - seams should be opposing, and be sure to get the center matched perfectly. Sew 1/4 inch on each side as before, cut and press open. You're done! So much easier than fussing with little separate triangles!
(If you only want 2 colors, you just pair 2 together.)



It should look like this above (although mine is not sewn).

Desired result on the back - I like to open up that center seam and twirl the seams. The seams on half the units will twirl in one direction, and the other direction on the other half of the units. These turned out completely accurate. I trimmed off the points.
If you are still a little shakey on this technique, sit down and practice. Some of us learn by reading, many of us by doing. I showed this to DH and he was totally amazed - as if it was magic!
(Whew - this took a lot longer to type out correctly than it did to actually sew it!)