Hi csi,

I think that you mean "twybill" or "bisaigue" which is the French equivalent name.

I own and have used one of these tools and these are not used to "make" a mortice but are used to clean out a mortice. Mortices are generally made by drilling a hole at either end and then using a morticing chisel to quickly and fairly roughly chop out the wood between the holes. The mortice corners are squared up and cleaned out using the morticing chisel end of the twybill which is then rotated through 180 degrees to use the paring chisel end to clean up the sides. Body weight is more than sufficient to power both ends of the twybill and being a long tool the timbers are best worked on the floor rather than on horses.

Regards

Ken Hume