A method of notating the octave range of an oscillator (analog or digital), borrowed from pipe organ terminology. The octave in which the oscillator will sound when middle C is played on the keyboard is designated by a number, which is the approximate length of an organ pipe which would produce the same pitch. An organ pipe (of "open diapason" type) of about 8 feet (2.44 m) will produce the pitch that is usually designated "middle C" or "C4" on a piano or organ; this pitch is about 261.6 Hz. ("Concert A", 440 Hz, is the A above this C.) Note that in the MIDI note numbering scheme, middle C is note number 60.
So, in theory, if the octave select on an oscillator is set to 8' (the notation with the apostrophe is an old convention for designating a measurement in feet), and middle C is played on the keyboard, it will play the same note that is obtained when playing middle C on a piano. If the oscillator is set to 4', it will play one octave higher; at 2' it will play two octaves higher, and so on. Whether this actually happens depends on number of factors, such as what other control signals (e.g., pitch bend) are being applied to the oscillator, the oscillator's tuning, and whether the keyboard is transposed. A typical analog VCO will play in tune and scale over a range of about five octaves, so the selection gives the performer an indication of where the useful low and high end of the range are; in order to play lower or higher, the octave selection is set to a different value.
Footage notations commonly used are 4', 8', and 16', and sometimes (mostly in modular synths) 2' or 32'. In pipe organs, other values noting non-octave intervals, such as 5-2/3' (a fifth above 8') are used, but these are seldom seen on synths.