Half square triangle units are squares made up of two triangles. Each triangle occupies half of the square's space.
You can sew two triangles together to create a unit, but it's easier, and usually more accurate, to assemble the units with quick piecing techniques that eliminate the need to handle individual triangles (with stretchy bias edges).
Quilters have developed several ways to make quick pieced half square triangle units, but my favorite is the technique I often call the 'easy sandwich method,' because two squares of fabric are sandwiched and then sewn together twice diagonally. When cut apart, the sandwich produces two identical HSTs.
The sandwich method is a perfect technique to use when you're making a scrap quilt or need a few HST units in specific color combos -- it helps you make great use of fabric scraps or small yardages.
However, I use this technique nearly all of the time. One reason -- I like to create oversize units and then cut them back to their exact unfinished size. That's not typically possible when using commercially pre-printed fabric or papers for assembly, another popular method. Trimming back is explained on page 5.
Want to see a visual of the technique? Watch our half square triangle video.
You might also want to try creating half square triangle units on a long grid.