Tall Paul:

There is no such thing as decalage on a monoplane.

It is, by definition, the incidence difference between (or among) two (or more) lifting surfaces.

Many misuse it, referring to the incidence difference between the lifting surface and the horizontal stabilizer, but that's incorrect.

Most common decalage is having the lower wing around +2 degrees, this makes the stall more gentle by having the lower wing stall before the upper.

By having the lower wing set negative in relation to the upper, the stall is more sudden, with a greater tendency to snap. You usually find this in planes designed specifically for aerobatics.

But remember please, there is no such thing as decalage on a monoplane.

Thanks.

Bill.