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 Jacob's Ladder quilt block fits into the easy category of quilt blocks because it contains only two shapes -- the square and half square triangle patches. It could also be considered an intermediate block due to the number of pieces in the pattern. It is a nine patch block, which is one of the most commonly used quilt block styles.
The sample quilts illustrate the use of four quilt blocks to make a single, larger quilt block. This allows for rotation of the individual four blocks to create a secondary pattern.
Similar Block ~ Rocky Road
(As a side note, Jacob's Ladder was one of the quilt blocks used to leave secret messages in quilts made to assist escaping slaves along the "Underground Railroad" on their path to freedom. The book Hidden in Plain View: A Secret Story of Quilts and the Underground Railroad tells that story of how these quilts were used -- which is quite a fascinating read.)