The simplest form of the Northumbrian pipes have a simple chanter and three drones. The chanter can play the eight notes of a single octave of a major scale.
The three drones are commonly called 'big' G, D and 'little' G.
This set of Northumbrian pipes is only intended to play in a single key and there is no real need for a mechanism to stop the drones.
The most common form of Northumbrian pipes has 7-keys on the chanter which extends the range of the chanter and allows two additional semitones within the original octave. To make use of these capabilities an additional low D, drone is added and a tuning bead fitted to the lower G drone allowing it to be tuned to A.
The 7-keys offer:
The fourth drone is generally called 'big' D and is an octave below the lowest note of the chanter.
Robert Reid further extended the capabilities of the pipes by the introduction of further semitones to make the chanter fully chromatic - 'The 14 key chanter'. His son James, lengthened the chanter taking the range down to B, below middle C with all the intervening semitones. This two-octave fully-chromatic chanter is generally known as 'The 17 key chanter'. Alongside these developments, tuning beads were added to the drones, allowing the pipes to play in other keys, even to the extent of modifying existing sets. James Reid also made a chanter pitched in the key of D, about a fourth below the G chanter.
Since this time, many other developments have taken place which will be described elsewhere.