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Block Lesson #3 // Skill Builder BOM

This post is part of a series of posts for the Skill Builder BOM (Block of the Month). To learn more about the block of the month or to see a list of all posts click here.

March Skill Builder BOM Technique

Piecing Triangles

This month we will be making our blocks using Half Square Triangles (HSTs) and Quarter Square Triangles (QSTs). Just as last month we will be working with bias seams. When sewing on the bias you must be very careful, because the bias stretches and can distort your block. This is where starch REALLY comes in handy. So please, please give starch a try. A few weeks ago, I posted a video tutorial on how to properly starch your fabrics, which you can find here.

The Blocks

This lesson is fairly easy and simple, so there was no need for a video tutorial. There is however a short lesson on triangles below.

Understanding Triangles

When you are working with triangles it is very important to understand what bias and straight grain means. This is because triangles can be can be cut in multiple ways to achieve the same block, but the block will be easier or harder to piece depending on whether you are sewing on the bias or not.


Let’s say you cut a basic square into two half square triangles (HSTs) (Figure 1). Each HST will have two shorter sides that run along the straight grain of the fabric. These sides are much stronger and have less give and stretch. Each HST will have one longer side running along the bias of the fabric, which is very stretchy and can easily be distorted. Cutting the triangles in this way can be best when you plan to make HSTs, because you will only be sewing on the bias once. That way when you piece you two HST units together you are sewing on the straight grain with less stretch.


To cut a half-square triangle and get straight grain on the two shorter sides of the triangle, take the size of the finished triangle (for this block, 4”), and add ⅞” for seam allowance (4” + ⅞” = 4 ⅞”). Now that may seem odd because usually we only need ¼” for seam allowances, but due to the angle of a triangle you must add ¼” for the seam allowance, plus an extra ⅜”. Once you piece two of the HSTs together you will have a 4 1/2” block, which will finish at 4” when completely sewn.


One of the fastest ways to make HSTs is to take two squares the same size and lay them right sides together. With a marking pencil, draw a line on the top square, dividing it into two HSTs. Stitch ¼” from your marking on each side (Figure 2). Then with a rotary cutter, cut along the line you marked (Figure 3). This will yield two identical HSTs that are ½” smaller than your original square. So if you had 5” squares, you will now have 4.5” HSTs. Using this method can help you to quickly piece HSTs and can help reduce some of the stretch as you sew on the bias. The only negative to this method is that it produces identical HSTs every time.


Now lets say you take a larger square and cut it into four quarter square triangles (QSTs). Each QST will have two shorter sides along the bias and one longer side along the straight grain (Figure 5). If you were to piece those QSTs together just as they are in Figure 5, you would end up with a block that has all four sides running along the bias. If you then needed to piece this block together with others like it, you may find it hard to avoid distortion.

You could also take two of the QSTs from Figure 5 and use them as HSTs and piece them together on the straight grain to create a HST block. You would end up with a smaller block with bias on all edges.


For this month's blocks, the triangles will be fairly easy to work with, but I do want to talk about properly lining up triangles.

In order to properly line up two triangles when piecing, begin by taking two HSTs (Figure 6) and folding them in half. Press on the halfway mark, leaving a visible crease. Now you can place both triangles right sides together by lining up your crease lines. This will align the triangles perfectly.

Once you sew the triangles together, you may find that they turn out to be slightly bigger than the final square you need. For example, if you were trying to achieve a HST block finished at 3”, you may find it finishes at 3.25”. You will need to take a ruler and carefully line up the seam with the 45 degree angle on your ruler, and trim your block up to 3” (Figure 7).


You will notice that triangles have long, skinny points. These points create what are often called “dog-ears” that need to be trimmed away, either before they are sewn or after they are pressed. I personally prefer to trim them off before I sew my pieces together. This is because I find it eliminates the problem of the points not feeding under the foot correctly and give you a straight blunt edge to feed under the foot as you begin to sew. These trimmed points also help you to line your triangles up perfectly without needing to fold your pieces in half. There are many different versions of these trimmer rulers out there. I am using a flying geese ruler with it, but also suggest Fons and Porter Trimmers.

Lesson Download

Go ahead an download Block Lesson #3 by clicking on the PDF button below. If you have any questions about the blocks, or see any errors, please stop by the flickr group or email me.

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Alyssa Williams

Alyssa Williams

Creator of Pile O' Fabric

Alyssa Lichner is a graphic designer turned avid sewist. She writes modern quilting tutorials and shares techniques and inspiration on her blog, Pile O’ Fabric. Alyssa has a passion for exploring different techniques and applying them to modern designs. She is thankful to have the opportunity to share these techniques, through her tutorials, patterns, and classes with quilters around the world. She always encourages her readers and students to challenge themselves to try new things and to approach quilting with a fearless attitude.