An extension organ is a pipe organ that uses one or more ranks of pipes longer then the length of its keyboards to serve several different organ stops at different pitches. This article is part of the Pipe Organ Refactor Project. ... An organ stop can mean either: The control on an organ console that selects a particular sound. ...

For example, a rank used as an 8' flute may also be used as a 4' flute simply by providing an extra stop control, to connect the keyboard to the pipes an octave above those that would normally sound. Using a single rank of pipes to support more than one speaking stop is generally known as borrowing, and occurs in several contexts other than extension. In this example, there are no pipes in the original rank to correspond to the top octave of the keyboard. In some cases of borrowing, borrowed ranks simply do not sound out of their ranges, and the player must allow for this. The other possibility is to extend the rank upwards by an extra twelve pipes, and the rank is then known as an extended or extension rank.

This approach still has two main disadvantages.

Firstly, there is the problem of borrowing collisions. In the example above, when both the 8' and 4' flute stops are selected and a key is pressed, say middle C, two pipes in the same rank will sound, the middle C pipe and the pipe one octave above it.

However, if both middle C and the key an octave above it are pressed at once, only three pipes will sound, as one has been selected twice, once as the 4' of middle C and once as the 8' of the key an octave above it. This is what is meant by a borrowing collision, and is one reason that borrowing a rank is regarded as inferior to having a dedicated rank.

Secondly, a dedicated 4' stop would be designed and voiced slightly differently; There is of course no opportunity to do this with a borrowed rank. An extension rank is in most cases voiced as a compromise. The more different stops it serves, the more compromise is required.

On heavily extended organs, some diapason ranks are commonly extended two octaves below and three octaves above 8', string ranks one octave below and one above, and flute ranks two or three octaves above. Extension is less commonly used for reed stops, one octave above to extend an 8' rank to 4' being the normal maximum, and very rarely for 64' stops, as large organs which have such large pipes tend to avoid borrowing altogether. Extension is not generally successful for mutation stops, as their intonation is incompatible. Intonation is the variation of tone used when speaking. ...

Borrowing between departments occurs in English organs from about 1700, but extension of pipe ranks for the purpose of borrowing at different pitches is a relatively recent development

Extension was heavily used in theatre organs in order to make the maximum use of a minimum number of pipes. Traditionally, less use has been made of extension in church organs and those designed for classical music, with authorities tending to regard borrowing in general and extension in particular as things to be avoided if possible. However some extension organs have been very successful in smaller churches. A theatre organ is an organ installed in a movie theatre, most often of a type originally devised by Robert Hope-Jones, which he called a unit orchestra. ...

An extreme example of a successful small extension organ is the instrument installed in Artarmon Presbyterian Church in 1962 by S. T. Noad and Son, and now moved to St Andrews Uniting Church in Longueville (both in suburban Sydney, Australia). Consisting of only three ranks (diapason, flute and string), it provides twenty speaking stops. At its dedication in its new location in 2002 it was warmly acclaimed by organ enthusiasts. Sydneys skyline with the Opera House on the left Sydney is the capital city of the Australian state of New South Wales and Australias largest and oldest city, founded in 1788. ...