It doesn’t matter if you are making a quilt with 4 HSTs (half square triangles) or 600 HSTs, it is very important that every HST is trimmed with the same trimming method to the exact measurement.
However many triangles you want to end up with is up to you, but remember that your squares for the HSTs need to be drawn 1” larger than the desired size block and sewn, then trimmed to the size needed.
Using the method below will get you two HSTs , but there are also methods for more HSTs depending on what you want to do! I recommend always doing a practice HST, so here we go!
The first step is to make two practice HSTs. Using two pieces of scrap fabric (one light and one dark is preferred), cut one 8” square from each fabric so that you have two 8” squares. On the wrong side of the light 8” square, use a ruler & pencil to draw a diagonal line from one corner to the other (fig. 1).
Next, put the two squares right sides together and pin on the placements (Fig. 2).
Then sew a 1/4” seam on both side of the diagonal line, following the arrows, if needed. Next, baste about 1/4” outside of the solid drawn square box, just to help hold everything together (fig. 3).
After you have sewn and basted, use your ruler and rotary cutter to cut the block in half along the line that you drew earlier.
+A NOTE ABOUT PRESSING+ It is important to press your HSTs and there is no right or wrong way to do it! Do what feels most comfortable to you. Pressing to one side: generally, you will press the seam toward the darker fabric if you chose this method. Pressing the seams open: this will generally allow your HST to lay extra flat, which can be an advantage when a project has a lot of intersecting seams.
Now it’s time to trim! This is a VERY important step. The secret to sewing accurate HSTs is in the trimming. There are different methods to trimming HSTs and whatever method you choose is good, however, just make sure you do the SAME method on EVERY half square triangle. It doesn’t matter if there are 4 HSTs in your pattern, or 600, it is very important to be consistent. Since you always add an extra 1″ larger than the desired size block and sewn, there is excess built into all of the cut square sizes to give you enough room to trim.
Here is a great tip: rotating cutting mats speed up the trimming process because you never have to pick up the HST to rotate it. Simply rotate the mat after trimming the first two sides rather than rotating the block itself.
1.After pressing the HST, lay it right side up on a cutting mat (Fig. 4).
2. Place your ruler on top of the HST, lining the 45-degree angle line on the ruler up with the HST diagnal seam. Position the ruler so that there is excess around the two open siddes, as well as excess past the trim sides in both directions (Fig. 5).
3. Trim along both edges (Fig. 6).
4. Turn the HST 180 degrees and line up the two cut edges with the trim size, aligning the diagonal seam with the 45-degree angle line on the ruler (Fig. 7).
5. Trim along both edges (Fig. 8).
In conclusion, you can make a lot of designs with HSTs, big or small!
The two pictures below are Becky’s method for making 8 HSTs at the same time! It uses the same method of drawing lines with a pencil, sewing and basting, cutting, and trimming to size.