Mortises with a marking guage

[Selwyn]

How do you mark mortices for mortice and tenon with just a marking guage (not a mortise guage).

I was shown how to do it years ago but forgotten.

 

[Racers]

By using two gauges or doing one side and then resetting the gauge and marking the other, I can't think of any other way and still marking from the face/edge side.

Pete