HSTs & QSTs: It’s Not You, It’s the Math

Have you ever wondered why points are so difficult to match up when you’re making a block with half-square or quarter-square triangles? It’s not so bad if the entire block is HST’s or QST’s by themselves (except that the block will most likely not finish to exactly the expected size), but pair either one with squares or rectangles and expect the corners and points to match up and everything to be sized right in the end, and you’re likely to be disappointed, no matter how carefully you’ve sewn or how accurate your 1/4″ seam allowance is.

Half Square Triangles

Well, I’ve got a news flash for you: it’s not you or your sewing skills, it’s the math. And I don’t mean that you or the pattern designer have miscalculated the cutting sizes either. What I mean is that mathematically, when you calculate the size of square to cut for a certain finished size of half-square or quarter-square triangle, you won’t get a size that’s rotary cutter friendly. It’s some algebraic formula like Asquared+Bsquared=Csquaredyaddayadda, and I did actually do the math a couple of years back though I don’t remember the formula at the moment.

This whole issue can be proven with graph paper as well. Try this: draw HST#1 with 2″ straight sides on graph paper, and then add 1/4″ seam allowances all the way around. Then draw HST#2 at the universally accepted cut size, with 2 7/8″ straight sides (the formula to determine the cut size for a HST is finished size + 7/8″). Measure the straight sides on the HST#1, which should be 2 7/8″, but they’re not. Sure, it’s not THAT far off, but when a block contains 8 or 16 HST’s, or when those HST’s are supposed to fit together with a 2 1/2″ square patch, this can cause issues down the road.

The bias edge across the middle of the HST adds its own measure of instability and inaccuracy, and it all equals wonkyness that’s not likely to be the right size in the end. What to do? I’ve gotten to the point where I cut large and trim down after the HST’s or QST’s are sewn, so that I know I’m getting the right size and everything is really square. I use the formula, then add 1/2″ or 3/4″ to the squares when I cut them. I dislike doing this for two reasons: one, it’s an extra step, and in multiples no less, since how many quilts need just one or two HST’s or QST’s, and two, it’s a fabric waster which is really not okay. There is a silver lining though (see, I can find one in this morass!): when you trim the HST’s or QST’s down to size, you’re trimming away the “dog ears” at the same time, so that’s a bonus.

Some patterns these days may just tell you to do that from the beginning, cut large and trim to the right size after piecing, but I’m basing all this on my personal experience with patterns and books, which is probably old since I don’t generally buy patterns or make quilts out of books anymore, preferring to tumble over the cliff of my own creativity into the scrappy mess of UFO’s and failed fabric experiments at the bottom. When I was buying books and patterns a few years ago, patterns were written based on cutting the “exact” size you need, and expecting the HST’s and QST’s to be the right size in the end and they never were, and the edges were curvy and wonky in the bargain.

So what’s your solution to the HST and QST finished size/cutting size quandary? Do you have these issues with patterns these days? Do you cut bigger and trim, or just deal with the fallout on the fly when piecing blocks?

Edit: Crazy Accuracy Freak Girl wrote this while I was having breakfast. 😉 I try to keep her penned up and away from the blog, but sometimes she sneaks out and adds her two cents…or two dollars, as the case may be. Thanks for putting up with her.—Nadine

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Driven to Quilt Podcast Episode 8: It’s all about accuracy: ways to improve your piecing and quilting accuracy; Barn Quilts: what they are and where to find them; and tips for hand appliqué.

Listen now!

Episode 8 program notes:

Rotary cutting accuracy tips1. Fabric that is not folded correctly for rotary cutting.


Rotary cutting accuracy tips2. Fabric folded correctly for rotary cutting, with no rolls or wrinkles.


Rotary cutting accuracy tips3. Trimming off ragged edges.


Rotary cutting accuracy tips4. Line up the fold with a ruler line.


Rotary cutting accuracy tips5. Check your fold and keep it lined up on a ruler line when cutting strips.


Rotary cutting accuracy tips6. If your strips have a “v” at the fold, the ruler was not lined up correctly at 90 degrees when you cut the fabric.


Rotary cutting accuracy tips7. Cutting angled pieces with the angled lines on the ruler.


Visit Scott Hagan’s website, and these other websites about Barn Quilts:
Monroe County, Ohio website
Ohio Quilt Barns
Quilt Barn Video
Quilt Barns in other states

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