This article describes the process and techniques involved in the tuning of a pipe organ. Electronic organs typically do not require tuning. A pipe organ produces sound via hundreds or thousands of organ pipes, each of which produces a single pitch and timbre. The goal of tuning a pipe organ is to adjust the pitch of each pipe so that they all sound in tune with each other. Pitch For many years, there was no pitch standard across Europe. The frequency of a′ (the standard note for tuning musical instruments), for example, could range from a′=392 Hz in parts of France to a′=465 Hz (Cornet-ton pitch) in parts of Germany. Organs were often tuned differently than ensembles, even within the same region or town. The modern tuning standard of a′=440 Hz (c′=262 Hz) was proposed in 1939, and accepted by the International Organization for Standardization (as ISO 16) in 1955 and again in 1975. Process The first task of an organ tuner is to select a temperament. Generally speaking, the temperament of a pipe organ is part of its design, and is not lightly changed during its lifetime. Equal temperament is very common, but by no means universal. Along with the temperament goes the overall concert pitch of the instrument, often A=440 Hz in modern instruments, but this also is far from universal. The pitch of an organ cannot be significantly changed without major work, as pipes need to be shortened or lengthened. Another important preparation step is to stabilize the temperature of the building in which the organ resides. Ideally, the temperature should be the same as that at which the organ will be typically used, and the temperature should have been stable for many hours before beginning the tuning. The reason for this is that the pitch of organ pipes vary significantly with temperature, and not all pipes vary at the same rate relative to temperature. The actual tuning process begins with the tuning of the "tuning stop", the stop to which most or all other stops will be tuned in turn. The tuning stop is usually the 4′ Octave or Principal (Diapason) in each division. The middle octave is usually tuned first, either by ear, or using some sort of electronic tuning device. The rest of the tuning stop is tuned to itself, in octaves. That is, tenor C is tuned to middle C, tenor D to