Mortise gauge

Nice!

Yeah... pretty simple. Some may say too simple and maybe not elegant but it works well and very efficient. Gauges are expensive so this route is attractive until you decide to put money out for a better one. 

Yep, pretty slick alternative to commercially made gauge, and this has an easier fine adjustment feature where the screw can be turned in or out a few degrees.

Oldtool.. the one thing I didn't mention is sharpening it so it cuts nice. First of all you use a slotted flat head screw. 
Then you take a file and back and forth across the top flat of the head until the circumference edge is thin and sharp. Also you don't want to file down to a point where the slot becomes useless. 

Sharpening the screw head for a cutting line, this gets better and better. It cuts, has extremely fine adjustment feature, and necessary parts are readily available in all wood shops while costing almost nothing. It doesn't get and better than this.

Oldtool.... AND.... if you use brass... there's no RUST!! 👍

Good on ya Mr. Rick!

Kind if a lesson in evolution here. Marking gauges have been around seemingly from day one.
I'd imagine they started out just like yours, then were tweaked and fancied up over the centuries into what is being sold now.
This brings it all back to a tool that was made to fulfill a need and fulfill it quickly.

Thanks SplinterGroup.... often it's "inspiration out of desperation" as a driving force. Right?
Often a need but no money to buy causes one to make it. 
I inherited some of my wife's Scottish great grandfather's chisels. Some of them were made out of files. It's odd to see the bevel beautifully shaped and honed for chiseling while the remainder of it still has the file cutting edges. 

Nice! That’s a great hack. 

Thank Ryan!  Check out the edge beading tool.