The information in the previous message is basically correct. Also, many sets of pipes were (and are) made without access to electronic tuning as a baseline which causes the whole instrument to be somewhat "off" A440. In addition, uilleann pipes are particularly sensetive to changes in climate--which causes subsequent change in pitch in the instrument. Pipers are constantly adjusting their reeds to stay in tune with themselves. Finally, some pipers play "flat sets" which were made deliberately 1/4 to 1/2 tone off--probably so other musicians could NOT play along so easily. Most bands or session groups tune their stringed instruments to the piper. Good luck playing along with a recording.