Wavy Edges on Your Blocks: Your Right Triangles May Not Be Right

A triangle by any other name would still be a triangle!
— Jan T (Sorry William S.)


Right Triangles Or The Right Triangles?

As I moved around in patchwork and quilting circles and after I started to teach people how to make quilts, wherever I went it seemed I was shown many blocks with ‘wavy’ edges, usually an Ohio Star block (below). The wavy (stretchy) edges of the blocks had bias grains in the triangles.

Quilting was in its infancy in Australia in the mid 1980s, and there were inexperienced tutors teaching others patchwork. The trouble was that we had no recent history of quiltmaking at that time, so the finer bits were unknown to most.

Of course I couldn’t tell the makers of those blocks that their tutor wasn’t teaching them properly! 

The first issue of Down Under Quilts magazine was released in March 1988, and I asked the then owner if she’d like an article about triangles in quilts. My writer’s career began with that article.

Why do you need to know so much about triangles in patchwork quilts?

About Right Triangles and Fabric Squares

  • In maths, a right triangle is defined as one which has one angle measuring 90°, and the remaining two angles add up to 90°.
  • In patchwork, there are two kinds of commonly used right triangles:  half-square triangles and quarter-square triangles.

Isosceles Right Triangles

While they are both the same shape in patchwork (they’re actually both isosceles right-angled triangles), it depends on how these right triangles are cut from your fabric, whether they are half- or quarter-square triangles.

To understand which is which you need to:


Start with a fabric square

 A square of fabric, if it’s cut properly, will have a straight grain on each side of it.

This is crucial to stop the edges of the block from being stretchy, and the quilt from being wavy.


Square of fabric

Half-square Triangles

If you cut a fabric square across on the diagonal, you get 2 halves — half-square triangles  which have two straight grains, the two sides that were the sides of the square, and on the straight grain.


cut on diagonal of fabric
Triangle of fabric

Quarter-square Triangles

If you cut the square with an X, you get 4 quarters — quarter-square triangles which have one straight grain, on the long side, which is the only side of the square that’s left.


Square of fabric
Triangle of fabric

What Kind Are They?

How do you know which kind of triangles are in the blocks you want to make?

Remember: A block Is A Square

It should finish with 4 straight grains at the edges, with the sides of triangles parallel to the block edge cut on the straight grain.

To Decide What Type Of Triangle

Examine the two blocks below. Triangles are in both blocks.

How many edges are at the side (or parallel to the side) of the block in the triangles you see?

Shoo Fly has triangles at the corners. What kind of right patchwork triangles are these?

Ohio Star has triangles at the centre of each side. Did you figure out what kind these triangles are?

The border on The Basket Quilt has many triangles. Test your new-found knowledge by working out which ones are half-square and which are quarter-square triangles.




Half Square Triangle
Quarter Square Triangle

Half- and Quarter-square Triangles in the border

Half or Quarter Square Triangles

It's Easy When You Find The Parallel Edge

Triangles in patchwork blocks must be cut with either two adjacent straight grain sides — half-square triangles, or cut with one side along the straight grain of the fabric —  quarter-square triangles.

If you follow this rule, you won’t have any more ‘wavy’ edges on your blocks.

Have fun! See you next time when I’ll talk about the rules for cutting both of these triangles if you are using template-free piecing methods.

Jan T

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