Regulators are pipes with a conical bore (similar to the chanter) with closed ends.
All holes are equipped with keys that enable the player to accompany the melody with chords. A Full Set Uilleann Pipes usually has a Tenor-, Baritone- and a Bass-Regulator.
A chart showing the position of the keys can be found here ##link##.
In rare cases, instruments are also equipped with an E- and a sub-bass regulator.