It's Time to Learn How to Sew a Quarter-Square Triangle


If you don't know how to make a quarter-square triangle, it is definitely time to learn. It is used in dozens of quilt blocks ( get an insight to quilt blocks in this class ) , and making one is a great skill for quilters of all levels of expertise.

First, some basics. A quarter-square triangle is a square fabric unit comprised of four right triangles. There are several ways to make one, but the easiest is starting with half-square triangles.

That's what I'm here to teach you.

1. Get Your Fabric Ready

There's a very simple formula for the size of the fabric pieces you need to make a quarter-square triangle: finished block size + 1¼ inches.

Once you have this info, gather at least two contrasting fabrics of that size. (I'm using Lily & Loom Dreamfield here.)

2. Make the Half-Square Triangles 

Place squares right sides together. Mark a line, corner to corner, on the wrong side of the fabric. Sew ¼ inch away from the drawn line on each side.

Use a rotary cutter and ruler to cut on the drawn line. Press seams toward the darker fabric.

3. Make the Quarter-Square Triangles

First, place the two half-square triangle units right sides together in the opposite direction (so the same fabrics do not stack atop each other). The seams should nest.

Next, draw a line on the diagonal that's perpendicular to the seam line (it will run across the seams). Pin in place.

Sew ¼ inch on each side of the drawn line. Then cut on the drawn line to create two pieces.

Finally, press the units open. 

Change Things Up!

A few simple swaps can completely change the look of your quarter-square triangles.

1. Mix-and-Match

Start with two different half-square triangle units to make a scrappy quarter-square triangle unit.

2. Half-Square + Quarter-Square

Swap out one of the half-square triangle units for a plain square. The result: two mirrored quarter-square triangle units.

Use this Cheat Sheet!

The instructions below tell you how to make a quarter-square triangle unit of any size.

What To Do If...

...You Have Wonky Units

The quarter-square triangle unit has bias edges, which stretch and become distorted easily. Starch can help the fabrics hold their shape. Using pins to sew the units will also avoid wonkiness.

...Your Units Are Too Small

The formula we use requires precise ¼ inch seam allowances, so double-check that you're sewing those seams accurately.

Seams pressed to the side can add bulk and shrink the units, so try pressing them open. (First, pin well to line up the seams.)  

Another option is to start with slightly larger squares than the formula calls for. Trim units to the correct size once they're all sewn.

Ready to Make Some Blocks?

1. Card Trick Block

Use four quarter-square triangle units, four half-square triangle units and one mixed quarter-square triangle unit.

2. Double Pinwheel Block

This one is comprised of four identical quarter-square triangle units.  Simply rotate each one to make the double pinwheel block.

3. Ohio Star Block

The classic Ohio Star block is made up of four identical hourglass blocks, plus five solid squares.

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It's Time to Learn How to Sew a Quarter-Square Triangle