Most of the pipe organs we maintain were built to be tuned to equal temperament at A=440, or in the case of instruments built in the early 20th century, A=435. The pitch of organ pipes varies in a systematic way with temperature, but somewhat differently for reeds and flues, so it is best to tune the instrument at the same temperature as it will be played.
The pitch of organ pipes is generally adjusted by means of collars, slides, scrolls, caps, stoppers, or tuning wires. All of these adjustments are affected by daily and seasonal cycles of temperature, which eventually cause the parts to move and the pipe to go out of tune. In wooden pipes, stoppers or slides are also affected by humidity. To prevent damage and to achieve adjustments of as little as a few thousandths of an inch, special tools are used for tuning.
Depending on the condition of the instrument and variations of temperature and humidity within the organ chamber, tuning is usually performed two or more times during the year. One of these may be a full tuning, while the others can often be partial tunings to correct a few notes which have drifted off pitch.
In a full tuning, the pipes of one rank, usually the Great Diapason or equivalent, are tuned individually using a pitch standard. Then the pipes of all the remaining tuned one by one to match the first rank.
If most of the pipes are already in tune, then a partial tuning can be performed. This is done by first checking and if necessary correcting the tuning of one rank, then testing whether the other ranks are in tune with it. Depending on the number of pipes needing to be tuned, this often takes less than half the time of a full tuning.
Tuning also provides an opportunity to discover and correct other problems, and it is helpful if the organist can provide us with a list of any known problems prior to the tuning.