Prior to the widespread adoption of the AGO pedalboard, there was considerable variation in number, spacing, and geometry from one organ to the next. The use of the standard pedalboard makes it possible for an organist to play an unfamiliar organ without relearning the pedal layout.
Other layouts exist and some are still built, especially outside the United States. The Malines pedalboard is a concave-parallel design, and there are two "standard" BDO designs--one concave-parallel and one concave-radiating. The concave-parallel design is the more common of the BDO designs. The BDO pedalboards have 30 pedals.
The B-3 Hammond Organ used a pedalboard with minimal concavity and only 25 notes, though an AGO pedalboard was available as an option.
Many organs sold for the home market have a scaled-back pedalboard covering only one octave (12 notes).