Double The Truth. Leitz Summicron 50mm DR

The last in the series of marking gauges in my tool collection; last, but not least.   
I bought this double gauge from Veritas just a couple of years ago.  It is meant for marking out mortise and tenon joints.  One would divide the thickness of the wood into  approximate thirds, two thirds forming the two shoulders of the joint, and the middle third, the tenon, and the mortise into which it fits.  One cutting wheel would be about a third away from the reference surface, the second about two thirds.  The distance between the cutters is set to the actual width of the chisel which is to be used to chop out the mortise.  What is left over, forms equal shoulders.  Did that make sense?
Veritas supplies an extra little clamp - visible in the background - for those that want it, to lock the two stems together to keep the distance between the cutting wheels constant, while adjusting their distance from the reference fence.  There is provision for both cutting wheels to sink back flush with the fence, so the tool can be used as a regular marking gauge.  It's a clever tool, beautifully made, and substantial in the hand.