How Not to Make Half-Square Triangle Units

  • Quick and Easy Isn't Always the Best Choice to Make Quilts

    The Wrong Way to Make Half Square Triangle Units
    HST Units the Wrong Way. Janet Wickell

    Beware of Some Quick Piecing Methods

    Half square triangle units (HSTs) are one of the most commonly used types of patchwork and there are lots of excellent methods you can use to create them.  But beware... some HST methods you'll see online are quick, but not good assembly options.

    One HST Assembly Option to Avoid

    1. Two large squares are sandwiched right sides together.
    2. The quilter sews quarter inch seams along each edge of the perimeter of the two sandwiched squares.
    3. After sewing seams, the large...MORE square is cut in half twice diagonally, just like we do when we cut setting triangles for on-point quilts.
    4. Units are pressed open along the seam lines, leaving stretchy bias edges surrounding their outside edges.

    Why Is This HST Unit Method a Problem?

    When we cut patchwork shapes and make patchwork units, the fabric's straight grain (ideally) runs along the outer image of the patches. Why? Because the fabric's bias is very stretchy and easily becomes distorted during pressing and sewing tasks.

    HST units have one edge (the longest) cut on the bias. Most HST quick piecing methods do not require us to cut and match separate shapes -- the long seam that runs along the bias is sewn before the units are ever handled separately. Sturdy straight grain edges end up on all four outer edges of each finished HST.

    With the method explained above, the stretchy bias runs along the four outer edges of each HST, and those edges are likely to stretch out of shape as you press, and as you assemble quilt blocks and the quilt top.

    A 'What If' Scenario

    Let's say you're making a basket quilt block or any other quilt block that's sewn with lots of HST units.

    • When you attach the HSTs to each other, you're sewing bias edges to more bias edges. I can pretty much guarantee that your rows will become distorted as you work on the block.
    • Baskets and many other quilt blocks have HST units around their outer edges, not just inside the block, so you'll have to deal with the extra stretch when joining quilt blocks, too.
    • Every time you press seam allowances or try to match up edges you run the risk of stretching those bias edges out of shape.

    Why Is It Okay to Cut Setting Triangles by Dividing a Square Four Times?

    The straight grain of setting (not corner) triangles must be cut so that the grain runs parallel to the longest edge of the triangles. Cutting a square with straight grain sides twice diagonally results in that grain orientation.

    You do need to be careful when handling any bias edge, but in most cases, the bias edges of setting triangles are sewn next to blocks or fabrics with straight grain outer edges. HST units with outer bias edges can create a stretch problem when sewn next to the bias edges of setting triangles.

    Take a look at an on point layout.

    What If I Want to Try the Method Anyway?

    Use starch, lots and lots of starch, to keep the fabric from stretching as much as possible. But keep in mind that the starch will wash out. Do you really want to run the risk of making a quilt that's not structurally sound -- one where those edges, though sewn, could stretch out of shape later?

    If You're a Beginning Quilter

    I've heard from beginning quilters who 'don't like to make HST units,' or 'can't make HSTs right.' In the majority of cases, these new quilters tried the 'sew around a square' method and didn't understand that their problems were caused by the method, not their sewing skills.

    Try some of the other HST techniques that are every bit as easy and produce stable patchwork.

    Other Quick Piecing Techniques Produce HST Units That Are Structurally Sound

    Bottom Line

    As a quilter, you will always need to handle bias edges -- there's no way to avoid it unless you sew with only square and rectangular shapes. But when ​there is a method that helps you sew stable patchwork, use it