You can look at it this way. These Triangles and Square are the same Finished Size and if you sew them together it will be the same seam line joining them.
If you look at the seam line on the Square, it is a Scant ¼" from each edge. So if you were to measure a Scant ¼" from the end of the seam line on the Triangle, that is where you would want to trim off the point.
The bottom line to all this is, the edges of Squares and Triangles that are the same Finished Size, should match exactly after you accurately trim the Triangle Points.
Patterns add rounded up seam allowance amounts for triangles because quilt rulers only have lines every 1/8":
Because those measurements are rounded up, so are the measurements used by most point trimmers. Most Point Trimmers use a width of .375" which is 3/8". But, if you do the math, the exact width you should trim at is .354".
The smaller the triangles,
the more that built-in error matters:
If you are cutting 3" or larger triangles, the built in error is a pretty small part of the total measurement and can be eased in fairly easily. But if they are 2" or less, it's a much bigger part of the total length.
So, when following a pattern precisely, unfortunately, Triangles will end up being slightly bigger than Squares of the same size no matter how accurately you cut them:
Now, there is a lot more that goes into accurate piecing beyond just cutting the pieces as accurately as possible, but who would want a built-in error if it can be avoided?
If you are following a pattern and something doesn't fit together right,
you may have no idea at what point it started going wrong.
Using Our Seam Allowance Additions and Prep-Tool
Eliminates the Built-in Error Completely:
Because the Seam Allowance Additions add the exact seam allowance amounts and the Prep-Tool trims the points at the exact spot, you can cut Squares and Triangles the exact same Finished Sizes and then see if the edges match.
Not only can you see how accurately you are cutting, but the Prep-Tool makes it easy to set a Seam Guide on your sewing machine at a Scant ¼". It also makes it easy to check how accurate your sewing is.