The bellows is held under one elbow (uilleann means elbow) and a leather bag under the other. The bellows feeds air into the bag through a one-way valve. The stock, a wooden cylinder, is mounted on the side of the bag. There are six pipes mounted in the stock. Three of these are drones: they provide a continuous note in three different octaves. The other three pipes are called regulators. They are closed and fitted with keys. They play notes only when the keys are pressed by the fingers or by the side of the hand. They can be used to play accompanying chords. The main pipe of the uilleann pipes, the chanter, is mounted at the end of the bag. It has a capped double reed. There are eight finger holes which are covered or uncovered to produce different notes.
The uilleann pipes are played sitting down. The end of the chanter is pressed against a small leather pad tied to the piper's knee. This enables the pipe to be completely closed, which allows the pipe to be silent. (This is impossible in the Scottish bagpipes). It is also possible by stopping the pipe to build up the pressure on the reed, causing it to switch into a different harmonic, allowing a second octave of notes to be played, so the uilleann pipes has a very large range of a full two octaves.
Most modern sets of pipes are pitched in the key of D, but in days gone by, lower pitches such as C and even Bb were used.