If you take a square and cut it across the diagonal from point to point. Then match that diagonal to another of a contrasting color or other color combination that you like, and put the good sides together and then sew along the diagonal. when you fold the now sewn together pieces out and press it flat you have a square made up of 2 different triangles that make up a square. I hope that is clear. enough. In the case of the squares I made they came out to be 1-1/2" squares. and it takes a lot of them to make up a large quilt. over 1500. They were very popular during the late 19th century and were known as 1000 piece quilts.
John