The half square triangle unit is probably the most commonly used patch in a quilt block -- well it is second only to the square. It is made simply with two triangles, filling up half the square each -- hence the name. Take a quick look through this website, or in any quilting magazine, and you will start to notice that this half square triangle, regardless of its size, is present in more quilts than not.
Free Quilt Block Pattern
The Swamp Angel quilt block is an adaptation of the Ohio Star block. By switching up the corner patches (from squares in the Ohio Star block), your design becomes a little more complicated, but still easy to sew. Squares, half square triangles and quarter square triangles are the only units used in this block.
Depending on how the quilt blocks are laid out, the focal points may be the stars or even on the secondary pattern that may emerge as you set the blocks side by side. You can see how this works by examining the illustrations for the sample quilts.