Please help with this pieced unit
 

I have been trying to make the units shown here using the method with 2 squares and 2 rectangles, but seem to have one side with a corner cut off. My mind has deserted me and I can't even remember a name to search for information. What is worse, the pieces of this quilt are mostly cut using eighth inch measurements, and no size given for any units, finished or not. I could have followed the patterns' instructions, but that square would be multi-colored - don't want that, so thought I would try it this way. Thank heaven the main section is finally done, this is the border. Love the quilt to pieces though.

Those look like side-setting triangles for an on-point quilt. Carol Thelen has a free on-point calculator that can help you determine the correct size for cutting those triangles. (You would cut 4 triangles from one larger square.) I suggest watching her video.
https://www.youtube.com/watch?v=umIEUEmNlAA

Just note: The Settings Triangle Calculator mentioned in the video has evidently been moved. I can't find it now. - sorry

Here is a calculator for setting triangles and blocks: https://files.allpeoplequilt.com/wp-...d_trianges.pdf To use this, take what your quilt block size is on the main top and use that measurement to find what you need for the other pieces.

i think this is the one the above poster is talking about. https://quiltnotes.com/pages/107-OnPointCalculator.php
the calculator for this one: https://quiltnotes.com/pages/106-OnP.../> Results.php

I would forget about using 2 squares and two rectangles -- basically I would use the calculator to figure out what size block I need and then I would sew 4 smaller squares together with the fabric colors you need, to make that large size square, then I would cut that square in half diagonally. Make sure to leave the seam allowance at the point where the full square and the two triangles meet in your picture when you cut that oversize (4 squares together square) diagonally.

Using paper, I would cut large square in half diagonally then put a square on point on top of it. Measure when I got the size unit needed, add seam allowance.

It sounds like you are describing a method for making a cat's cradle unit. Your photo shows one half of the unit, then you'd typically add the large HST to complete the unit. But if these are to be your border, you're not making the whole unit, just the part shown, which I've seen called a 'wing unit' by Bonnie Hunter in some of her mysteries. I've only made this with a square and two triangles -- can't quite picture how using squares and rectangles together would yield the unit shown.

It's the pieced half of a shaded four patch unit. If you search for shaded four patch on whatever search engine you prefer, you should find several links. There are both written and video links.

I do a lot of cut large and trim to fit. In this case I would cut the square correctly and make the side triangles a little large, Just sort of center the oversized triangles. You don't need a whole lot of extra, just enough to make sure your sides and bottom are full, so if you care cutting the sides as a square, make it 1/2" larger (so 1/4 for each of the triangles).

For trimming, the top part is easy. Just use a corner/square ruler for the top -- you should not be trimming off any of the square, just the triangle sides. For the long cut on the bottom, I would mark my ruler using tape. Again, using the square as correct I would position the ruler and just do a fast cut along the bottom.

Flying Fish (quilterscache.com)
Part of this block has your unit. I hope this helps.

https://quiltsbyjen.ca/grandmothers-choice-quilt-block/

if you scroll through the instructions for this block, maybe it might have some info to help?