Half square triangles and quarter square triangles are both commonly used triangles in many quilt patterns and designs. Toby Lischko explains what the difference is between the two, how they are cut and where they should be used.
Half Square Triangles
A Half square triangle, which is commonly abbreviated HST in most quilt patterns, is a triangle that was cut from a square. Traditionally one will start with a square of fabric the size specified in the pattern, and then cut it in half diagonally once to get two half square triangles. HSTs can also be made in other ways, including making half square triangles with jelly rolls. HSTs can be used in several different ways. One way they are commonly used is in units that make up a block, which is taught in this tutorial on half square triangle piecing.
Another way half square triangles are used is in a quilt design that is set on point. Because a HST is made from cutting a square in half, two sides of the triangle will have straight of grain sides and one will have a bias edge. The HST can be used in the corners of an on-point design to ensure that the outer edges of an on-point quilt will have straight of grain edges.
Quarter Square Triangles
Quarter square triangles, which are common abbreviated QST in most quilt patterns, are triangles that were again cut from a starting square. You will begin with a square of fabric the size specified in the pattern and then cut it in half diagonally twice to get four quarter square triangles. QSTs are commonly used in units that make up blocks and also in on-point quilt layouts as well. Because of the way they are cut, they have two bias edges and one straight edge. Using a QST as a setting triangle on an on-point layout will again ensure that all of the outer edges of the quilt will be straight of grain edges.