It has become the accepted wisdom that Northumbrian pipes are played with three drones sounding. 2 drones are an octave apart and the third is pitched at the dominant between the other two. The drones thus have stoppers or plungers to switch individual drones off. Further, the drones can have tuning beads which allow the pitch of the drone to be raised by a tone. In the simplest case a set of 4 drones will have a one tuning bead on the second largest drone.
More complex drones have at least one tuning bead on each drone.
When playing with other musicians, many pipers do not use their drones at all. This frees the piper to change key along with other musicians, and anyway, the subtleties of drones are often lost when playing with other musicians. Northumbrian Smallpipes and Highland Pipes the differences. Northumbrian Smallpipes are much, much quieter than Highland pipes. Northumbrian Smallpipes are bellows blown, and they play at much lower pressure than Highland pipes.