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.
Mrs. Keller's Nine Patch
Free Quilt Block Pattern
Mrs. Keller's Nine Patch is an interesting name for this quilt block because, depending on how you look at it -- (and your colour choices for fabric) -- it contains a nine patch block in the centre (although you will really have to search for it), or the construction (as I have shown you here) is to make this quilt block as an "uneven" nine patch block.
This quilt block could be considered as a five patch as well, because it is made on a 5 x 5 grid. All in all, this is an easy block to construct and you have great fun with your colour choices.