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: http://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.