How to quilt half square triangles

Half Square Triangles in Patchwork Blocks

How to quilt half square triangles is one of the shapes most often found in patchwork quilt blocks aside from squares is the half-square triangle. A half-square triangle is basically a triangle that is made by dividing a square into two halves on the diagonal. They have one 90-degree angle, which makes them a right angle triangle. Two of these triangles are joined to make a half-square triangle unit. You will find these units named HST when reading quilt pattern instructions. It’s a versatile unit, mixed with other shapes or repeated to create patchwork blocks and also to make interesting borders.
Quilt blocks such as Ocean Waves are composed entirely of half-square triangle units. It’s a simple shape to sew, but it makes a most impressive quilt when the blocks are sewn together. It lends itself very well to using scraps.

The one important consideration when cutting and sewing half square triangles is to keep the bias in the center, along the diagonal that goes from corner to corner and avoid bias edges along the sides of the unit. This is particularly important if the half-square triangle is going to be positioned on the edge of a patchwork block. Bias distorts easily and, if on the edge of a block, it would make the block ripple and possibly end up stretching when pressed. This in turn would make an unstable quilt top that wouldn’t lay flat.

It might seem complicated, but by using quick cutting and machine piecing methods, cutting half-square triangle units is a snap. There are several ways to produce this particular unit that yields fast and accurate results. You will need to have two squares of fabric of contrasting color, sewing machine threaded with cotton thread in a neutral shade, a rotary cutter, mat and a square ruler that has a 45 degree line.

Method one: Cut two squares of the contrasting fabrics needed to piece the unit. Cutting measurement is finished size of the unit + 7/8 inch. So if the half-square triangle unit is to measure 2 inches finished size, you will need to cut squares that are 2 7/8 inches. However, I prefer to add one inch to the finished size, then trim them to size, as the 7/8 inch increase can yield units that are slightly too small. It is always easier to trim than to find units that are not quite large enough.

Join the two cut squares right side together, and with mechanical pencil, mark a line from one corner to the opposite on the lightest of the fabrics. Then, either mark a line exactly one quarter inch away from it on both sides of the drawn line, use your quarter inch foot as a guide, or eyeball it, if you feel confident enough. Sew on both the lines. By sewing before cutting the unit apart, the fabric is more stable and less likely to stretch or distort.

Give it a slight press, and cut on the first line drawn. Now you have two units composed of two half square triangles. They need to be pressed down with the seam allowance towards the darker of the two fabrics, and trimmed to the required size, lining up the edges with the markings on the ruler and the center seam with the 45 degree line on the ruler. Once again, if the finished unit size is two inches, you need to trim the unit to two and a half inches, to add seam allowances.

This method works well if you need a lot of different color half triangle square units, or if you are working with fabric scraps. You can chain piece these units on the sewing machine, that is, sew seams without cutting the thread. Then, when you have several sewn in a chain, snip the thread chain connecting them, press them and cut them apart.

Method two: If you need several half-square triangle units from the same fabrics, you can sew eight units at once by cutting two squares of two fabrics measuring twice the measurements used above in method one. So, for instance if the half-square unit needed is two inches finished, you will have to cut one six inch square:
Finished half square triangle unit size = 2 inches
Add 1 inch = 3 inches
Multiply by two = 6 inches

Draw lines on the light fabric square as indicated on the picture. The red lines are cutting lines, the black ones, sewing lines. Sew on the drawn black lines, press and cut apart on the red lines. It will yield eight half-square triangle units. Press seam allowances to the darker fabric and trim so that each unit measures two and a half inches.

One of the simplest blocks one can create with the half-square triangle units is the Pinwheel block. It is composed of four units. First, sew two HST and press seam towards the dark triangle. Then, sew those together, interlocking the seams at the center in order to get a nice, crisp point when pressed.

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