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Half Square Triangle Sewing Notions

Half Square Triangles are one of the most common blocks used in quilting and it’s worth having a few tricks up your sleeve and specialty tools on your cutting table before you start sewing!

My favorite ways to make HSTs require a few specialty tools, I’m all about making things easy for myself, but you don’t need any of these things to make them.

I’m a self-admitted gadget girl and I can’t help but try all the things. In an effort to save you money and make my craft room clutter serve a purpose, I’m going to walk you through my must-have tools for sewing half-square triangles and give some options for alternatives when available.

1/2 Inch Ruler

This may sound like a really strange pick but all of my HSTs start with this ruler. It’s just a 1/2-inch wide piece of plastic used to mark across the diagonal of fabric squares.

10" fabric square laying on a rotating cutting mat with a 1/2 inch ruler and a green frixion pen

All you need to do is line up your 1/2 Inch Ruler across the diagonal, draw lines on both sides and then sew along them (I prefer to sew just inside them). If you have a bunch to make this is a great time for chain piecing!

Once you’re done sewing you cut down the diagonal between your stitch lines, and press the HSTs open. This tool is really useful for both the 2-at-a-time and 8-at-a-time sewing methods, they’re pretty much all I use at this point so I never make an HST (or half square rectangle!) without this tool.

You can get a similar effect using a regular quilting ruler, you’ll just need to draw one line and then switch the ruler to the other side to draw the second. It doesn’t that much time but anytime you’re doing something 100’s of times any way you can make it faster is a good bet!

This tool is also really helpful when you’re making half rectangles, I know I reached for it when I made my Black Diamonds quilt!

Get well acquainted with Half Rectangles with the free Black Diamonds Quilt Pattern

Frixion Pen

Going back to drawing those lines, you can draw your lines with chalk or a pencil but I love using Frixion Pens. They’re filled with ‘erasable’ ink that disappears with heat, like from an iron.

two squares with diagonal lines and two stitching lines to make half square triangles

It’s important to remember that the ink will reappear if your quilt gets cold and some of the colors leave ghost lines behind. I only use them on the back of my blocks but I really don’t think I could do half the sewing I do without them!

I usually buy giant packs of them in all sorts of colors during the back-to-school sales. There are a few different styles, I prefer the fine point because you can get closer to the ruler but they do seem to dry out faster.

BlocLoc Ruler

The most tedious part of working with half-square triangles is trimming them up. As much as I hate to admit it, you really need to square up each individual block.

halloween fabric Half Square triangle block with BlocLoc ruler on top for squaring up

Trust me, if you could skip this step I would! Instead, I throw on a podcast and reach for my BlocLoc ruler.

They come in a few different sizes but I have the 6.5″ version. I’d say 95% of the HST blocks I make are small enough to trim up with that size.

The Bloc Loc rulers are available in a variety of sizes, from 1.5″ all the way up to 12.5″ and they do get spendy when you look into the larger sizes

Bloc Loc rulers have a groove across one diagonal that you nestle up to the seam, they’ll only work as intended if you press your seams to the side and not open.

That groove literally locks in the seam and you can slide the ruler around to get the perfect spot for trimming. After you trim the first two sides, leave the ruler in place on the fabric and spin it 180 degrees so you can trim the other two sides.

Clearly Perfect Slotted Trimmer

The Clearly Perfect Slotted trimmer is another specialty ruler used to square up half-square triangles. Unlike the Bloc Loc ruler where you press the blocks open before trimming, with the CPSL you trim the blocks down before opening them up.

The main benefit is that it’s faster, you’re only going to be making 2 cuts instead of 4. The rulers also have little slits you can use to trim off the corners.

I do not have a great track record with these rulers, I think my main issue is I don’t sew particularly straight lines. So when I trim and then open the blocks up they aren’t perfectly squared.

Personally, I also find the little slots awkward to get my blade into and it’s just not for me. There are a lot of people that swear by them, you might be able to reach out to a friend to try them out before you buy them.

And I’m specifically saying them because you’ll likely need more than one. I have a set where one ruler can be used to trim whole-inch squares (2″, 3″, 4″ 5″, and 6″) and the second is used to trim 1/2 inch squares (1.5″, 2.5″, 3.5″, 4.5″ 5.5″ and 6.5″).

You need to make sure you have the right ruler for the size you need and you need to double-check you’re using the correct slots to trim off your corners or you’ll accidentally cut slashes in your block and need to start over.

If you just can’t get enough half-square triangle action be sure to check out:

Rotating Cutting Mat

The last quilting tool I’m going to talk about might be my favorite. A Rotating Cutting Mat is basically a Lazy Susan with a cutting mat on top.

It allows you to trim one or two sides and then spin the mat to make the following cuts without disturbing or misaligning your fabric.

If you struggle to draw on both sides of the 1/2-inch seam guide the rotating cutting mat makes it much easier to get to the other side.

I find this really helpful when using the 8-at-a-time method, making the + and x cuts are a lot easier when you can move everything at once!

It also comes in handy if you’re squaring up blocks with a regular ruler, it’s easier to spin the mat than the fabric.

I do not recommend using the Bloc Loc ruler on a rotating cutting mat, they tend to work against each other so just let the ruler do the work and save the spinning cutting mat for another use.

Check out my Quilting page for more info or start here:

cozy corners quilt sewn with red, green, cream and gray flannel displayed in a tree surrounded by snow with text overlay "cozy corners layer cake quilt pattern"