Rick you beat me to it. I love Meghan McMillian's Magnificant Mat Marker! I believe they are around $20 plus shipping. Contact her for details.
Luke, I know the owner, used to work for him. I do not know if Bill has any left. I have a few hidden somewhere, but not willing to part with them.
Give him a call or email, https://www.archivalartservices.com/ Tell him Dennis Yankow sent you. He has made many neat inventions in framing. Good luck.
so you're marking and cutting on the face of the mat?Yes, the lines on the corner are to make double mats. You cut the top mat first. When you size the bottom mat, put the mat with opening on top of it. Put the triangle in the corner, then you can make a line depending on the width you want to use.
Sorry, if that was muddled. It is easier to do than explain. I was taught by Bill and Matt, head hinger/matter.