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Using Triangles in Patchwork Quilts

Updated on January 21, 2017
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Quilter, author, tutor, and columnist, Jan T Urquhart Baillie has been sharing her passion for quilting for more than 30 years.

All About Triangles in Quilts

The importance of straight grains when cutting triangles for patchwork quilts cannot be emphasised enough.

What Kind of Triangle?

A triangle by any other name would still be a triangle! There are equilateral, isosceles, right, scalene...

How Are Patchwork Triangles Different?

In patchwork, there are two kinds of commonly used triangles: half-square triangles and quarter-square triangles. In the quilt Singing the Blues, at right, both quarter- and half-square triangles were required in the solid blue fabric.

Actually, from a maths point of view, these are both right triangles and usually isosceles.

Why Does It Matter?

In a patchwork block, the ideal is to have the straight of grain at the edge or parallel to the edge of the block/quilt.

My very first patchwork article was on this topic, because people kept showing me blocks which were stretchy on the edges, because some tutors didn't understand why it mattered.

So why does it matter?

If the block edges are not on the straight grain there is a danger that the blocks may finish at all different sizes and the quilt will not hang well. The other problem that occurs is that the edges are 'wavy' and hard to control when joining them to the next block, or when attaching the borders.

I decided to write an explanation for patchworkers, and that's how I got my first writing gig at Down Under Quilts magazine!

Triangles In Patchwork

Should have the straight grain parallel to the edge of the block or the quilt in order to have the quilt lie flat.

Straight grain on all sides of a square of well cut fabric
Straight grain on all sides of a square of well cut fabric

Straight Grain is Important

It keeps the quilt straight.

How does this work?

To understand, you need to start with a square of fabric.

A square of fabric has four straight grains — one on each side.

Or it should!

It is important to cut carefully so that your patches are on the straight grain of the fabric, either crosswise grain or lengthwise grain.


To see what this means in practice, here is a traditional block called Shoo Fly.

One of the triangles in the corner is marked with the correct grain lines.

Cut square of fabric in half diagonally
Cut square of fabric in half diagonally

Half-Square Triangles

These Have Two Straight Grains

Start with a square of fabric.

If you cut the square across the diagonal, you get two halves, that is, 2 half-square triangles.

Rule for ½ Square triangles

Cut a square of fabric whose length =

finished side of square + 7/8"

Cut this square in half diagonally.

Triangle squares

These units are ...

... what you get after you join two half-square triangles together.

That would be how the units in the corners of the Shoo Fly block above would need to be constructed.

Cut square of fabric with an X
Cut square of fabric with an X

Quarter-Square Triangles

What Are They?

Quarter-square triangles have one straight grain on the long side, and two bias edged (stretchy grain) sides.

To see how this works, start with a square of fabric.

If you cut the square with an X, you get 4 quarters — quarter-square triangles, which have one straight grain on the long side — being the side of the original fabric square.

These triangles have one straight grain side: the side of the starting square.

They also have two bias grain sides: ve-e-e-ry stretchy!

Rule for 1/4 square triangles

Cut a square of fabric whose length =

finished side of square + 1¼"

Cut this square with an X

Ohio Star
Ohio Star | Source

Ohio Star Blocks

The traditional Ohio Star block has quarter-square triangles only and is shown here with the grain marked on the quarter-square triangles, at the edges.

It is essential that these are triangles with the straight grain at the edge of the block, otherwise the edges will be ruffly and hard to use. The side of these triangle patches is one third of the side of the entire block, so it is very important.

When you make the Ohio Star block (above, right), you'll need four of the units (right), each made using four quarter-square triangles.

Each coloured triangle in the diagram is a quarter of the square that they make when sewn together, and therefore they need to have the straight grain on the long sides.

How else are triangles used

in patchwork quilts?

Apart from constructing blocks and borders, triangles are used to turn blocks on point, or to set blocks on point into an entire quilt setting.

  1. For turning blocks on point
  2. For setting blocks into on point quilts

The quilt at the top of this page is set with triangles.

1. Turning Blocks On Point

How to Calculate the Size of the Triangles

In my book Listen With Your Eyes, there is a quilt by one of my students (Anne Archer) using many blocks set into triangles (Moody Blues).

If you want to use a block on point in a straight set quilt, you need to sew triangles to the corners.

How It Works

For turning blocks on point to set them in straight set using rotary cutting, you need to be able to calculate the size you have to cut.

Here is What You Do:

First, you need to establish the diagonal measurement of your block when finished, e.g. 6 inch block.

The mathematics come from Pythagoras' theorem about the square on the hypotenuse and so on, but it is simple.

Maths? Don't Worry

It's easy my way.

Close Enough!

You say that this diagonal is about 1 ½ times the finished side of the block, for a 6" block, that will be 9".

In reality it is 6 x 1.4142, but 1½ times is close enough!

Next Step

Now you need half that measurement, because you are only putting triangles on the corners.

I like to call these corners 'imaginary squares' because that is what 'appears' once the corner triangle is sewn on. (See the diagram at right.)

So for our example, you need 9 divided by 2 which is 4 ½,

Apply the Rule

Next you apply the rule for half-square triangles which is 'add ⅞" to that finished size'

  • So 6" x 1 ½ = 9"
  • Half of 9" = 4 ½"
  • Add ⅞" to 4 ½" = 5 ⅜"

Cut two squares this size and cut in half diagonally to get all four corners.

It's the same reasoning for any size block and the sides are then all on the straight grain.

It's a Triangular Mystery

Triangles of both types are used in this Mystery Quilt.

Can you see which are half-square and which are quarter-square triangles?

Lots triangles in this quilt
Lots triangles in this quilt

Sewing Curved Seam Triangle Squares (A Request From Jenni)

Was this lens helpful - in your patchwork and quilting?

Did you learn anything about triangles for patchwork?

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© 2009 Jan T Urquhart Baillie


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