Which of the following instrument does not have 4 digits?
GMT
Excavators
Enamel hatchet
Three-Number Formula

It is used for the instruments in which cutting edge is at right angle to the long axis of the blade.

Eg: Enamel hatchet
Four-Number Formula

It is used for the instruments in which primary cutting edge is not at right angle to long axis of the blade.

Eg: GMT, excavators