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
Air Castle is a nine patch quilt block containing half square triangles, three-quarter square triangles, and a square in the square unit for the centre of the quilt block. This is an easy to intermediate skill level quilt block pattern.
The sample quilt shows the blocks laid out side by side with several borders. A totally different look can be achieved by simply switching out the colours in the borders.