Quilting triangles are usually made by
cutting up squares. There are a couple reasons for this. Usually you
need two triangles, so there isn't any waste. However the main reason is
that you want to avoid
handling the bias as much as possible.
If you want a half square triangle, you cut a square
once diagonally. If you want a quarter square triangle, you cut the
square diagonally twice.
Half square triangles have the bias on the
long side.
Quarter square triangles have two bias edges on the short
sides.
The following instructions are for making half
square triangles.
The pictures below are of the Spool and
Bobbin quilt square. The Spool and Bobbin is a 4patch quilt block that has
been further divided into 16 patches. As you can see from the picture, each block has 2 large squares in the corners that cover 4 of the smaller
patches.
The other 2 large squares contain the smaller
patches. There are some shortcut methods to make these blocks, but for the
purpose of cutting triangles we are going to do it the long way.
Focus on the top left quadrant of the square.
You see a small square in the corner that has small triangles on each side of
it, and a larger triangle on the bottom. Here is how you calculate the
measurements for these pieces and put them together.
Beginning with the small square, you can see that
it fills just one of the smaller grids. Planning each small grid to finish at 2
inches, these small squares are cut at 2 1/2 inches. This is half of the
measurement of the large yellow squares, which are cut at 4 1/2 inches.
To calculate the size of the large blue
triangles, take the finished size of the large square, and add 7/8 inches.
Then cut it in half diagonally for 2 triangles.
To calculate the size of the small yellow
triangles, take the finished size of the small square and add 7/8 of an inch.
Cut it in half diagonally for 2 triangles.
When you added 7/8 of an inch to the size of the
square, 1/2 inch is for the seam allowances.
The additional 3/8 of an inch is for the seam between the triangles and the
piece you will sew them to.




Triangles cut for upper quadrant 
Small sides attached to square and
unit attached to large triangle 
When attaching the small triangles to the square match the outer
edges. The pointed part of the the triangle will extend beyond the seam
line. Attach one triangle, press, and the do the other one. Trim
edges even as you go.
When attaching the top unit to the bottom triangle, keep the top
unit facing up when sewing so that you can see when you get to
the middle of the seam. You want to sew right in front of the stitching
line so
that you do not cut off the point of the small triangle when it meets the large
triangle.
You will not get into trouble cutting triangles as long as you
remember the difference between raw and finished measurements, and use the 3/8
or 7/8 inch measurements.