|By chance, I experimented with a way to keep my buttonholes straight.
I just finished a jacket made from silk tweed. Because of the various colors in the fabrics, marking the line with chalk was not possible.
Further, the buttonholes needed to be long to accommodate 1 1/2" buttons.
It dawned on me that I could baste the line with long stitches in a bright color. I expermented a couple of times and was happy with the results!
The basting stitches start out straight and give a strong guide in which to follow and it doesn't rub off from handling. It is then simple pulled out of the center when slashing.
This technique could be used for any buttonhole, not just on busy, hard to mark fabrics.