I learned about this triangular shaped bound buttonhole in 1993 when I bought the book
Couture the art of fine sewing by Roberta Carr. I have made these kind of triangular buttonholes several times , the one above was done in a raincoat .
I made an example to show you the steps how to make such a triangular buttonhole and kept my text to a minimum because of copyright issue.
Step 1, measure the button length and space between the edge and the buttonhole.
Step 2, mark and transfer the lines ( horizontal and wide) on the interfaced fabric .
Step 3, thread trace the buttonhole length and wide on the patch.
Step 4, draw the triangular button lines.
Step 5, stitch using a tiny stitch length start anywhere on one of the long lines and stitch 2 diagonal stitches at the corners and at the point, overlap the beginning of the stitches with a few more stitches. I used a red thread for easy viewing.
Step 6, draw lines so you can slash the patch in 3 sections . The lines of the patch marked in red, cutting lines inside the buttonhole in black.
Step 7, turn patch section 1 to the inside and press .
Step 8, press patch section 2 along the stitching line.
Step 9, do the same with patch section 3 as above.
Step 10, wrap patch section 2 and 3 around the horizontal slit edges on the inside and press from the inside .
Step 11, stitch the triangular tab to the patch through all the layers so it will be permanently attached.
Step 12, cut away excess fabric of the patch.