Some folks who chop a lot of mortises don’t even bother with a mortise gauge, instead marking one line with a standard marking gauge. I find them handy, and on most seventeenth-century pieces I have observed show a pair of lines. Sometimes these lines are staggered, the starting points being offset. That can mean that the pins that struck the lines are not on the same beam, rod, staff, whatever you might call that part of the gauge. Or it could be 2 settings of a marking gauge instead of a mortise gauge.
So here’s another in a series of mortise gauges, this one sent up from Alexander’s collection. It is self-explanatory really. Two round rods with screws set in them, captured in a split head, which is then screwed closed around the round rods. Simple as can be.
Here is a detail of the head, showing the staggered positions of the marking pins:
what this gets you is a pair of lines marked on your stock that do not begin & end at the same point, similar effect to one of the mortise gauges posted here recently: https://pfollansbee.wordpress.com/2011/08/12/two-nice-mortise-gauges/
This next view shows the nature of the head or block of this tool – I assume it’s a single piece of wood, bored for the rods, then sawn apart. then screwed back together. Makes sense anyway.
PETER FOLLANSBEE: JOINER’S NOTES 
So it is just plain woodscrews that hold the two halfs together? Here in Sweden I see a lot that have a captive wedge between two (square) rods.
Any idea as to the age of the one you are showing here?
Looking at the wear pattern, would I be correct in thinking that it has had a lot of use as a normal marking gauge, with one of the arms twisted to point along the join? If so, then this would make it more versatile than my modern mortise gauge. Or maybe each side of the mortise was marked individually?
Now I am very intrigued as to how it was used.
Paul
it seems like the gauge was used the way you thought, that it can function like a marking gauge, in addition to being a mortise gauge. I can see where you might set one staff for say a 1″ marking, and the other for 3 1/2″ – if you were preparing a lot of stock for joined frames…so set it up, & no fiddling around. It might be worth making one.
I never cease to be amazed at the ingenuity of these early artisans, this is such a simple idea, for a very useful item. I am definitely going to make one.
Peter
Another Mortise Gauge is a fascinating creature. First, I feel Paul Cooke and your Commenta are right on. Indeed this devise can readily be converted into a wide range single marking gauge by simply rotating the “inner” pin on the short beam 180 degrees so that pin is out of action. It is doubtful that the “inner” pin on the shorter bean was slid back out of the way. If so the fence’s surface would be scratched. With the “inner” pin out of the way, the “outer” pin on the long beam then becomes the operating pin for a regular marking gauge. The use of rotating dowels is the heart of the matter. Rotate the long beam 180 degrees having another pin say 5/16ths away would offer a fixed mortise gauge on the projecting beam any distance from the shoulder. Indeed it appears possible that another pin went through the long beam opposite the other. As to age I doubt that this gauge is 17th-century. The use of metal scrws to clamp the dowels and the use of dowels themselves suggest otherwise. Popping the screws out for examination might assist conjecture. Who cares! The idea is better than sliced bread.Yes Paul, versitile, simple to make and much, much cheaper. Paul if you make one, perhaps you would report to Follansbee and even send a picture. Oops, one thing else, I’d suggest drywall screws for pins. Possibly bolts and captured nuts for the fence.
Jennie
Consultant
Wonderfilled Ersatz Tool Department