At times Moog Music sold certain pre-packaged configurations of their modular synthesizers, with certain types and sizes of cabinetry. All of these were customizable by the customer, and it is likely that no two of a particular model actually left the factory with the exact same configuration. It was also possible for a customer to expand an existing synth by adding more cabinets. Nonetheless, the model designations served as a useful shorthand for the general size and scope of a particular unit.
Prior to 1967, there were no model designations; all modular synths were custom built. Starting in 1967, Moog introduced three standard configurations, designated by the Roman numerals I, II, and III, followed by the letter 'C' indicating a "console" configuration intended for studio or institutional use, or 'P' indicating a "portable" configuration for stage and touring use. The 'C' models were based on a walnut base cabinet with a sloping front, 22 module units wide, and containing one module row above one MCP row. In the IC configuration, the module row was partly filled with modules and partly with filler panels; in the IIC configuration, it was completely filled with modules. The IIIC added a rectangular (vertical front) "upper cabinet" which sat on top of the base cabinet and was filled with additional modules. The 'P' models were based on a portable and road-worthy (in theory), vertical-front cabinet which contained two module rows of 8U wide, plus an MCP row. The model IP used one such cabinet; the IIP two, and the IIIP three.
In 1970 Moog introduced two minimally configured, lower cost models designated as the Models 10 and 12; these were based on the P cabinet. In 1972, all of the existing models were replaced by the Models 15, 35 and 55. The Model 15 used one 'P' cabinet; the model 35 one 'C' base cabinet, and the model 55 used a 'C' base cabinet and an upper cabinet. Some pre-configured upper cabinets were available as expansion units, notably a cabinet that contained two model 960 analog sequencers and associated modules. The introduction of the models 15/35/55 coincided with the substitution of the original model 901A/B voltage controlled oscillators with the more stable model 921 VCOs.
Some sources also make reference to a model known as the Coordinated Electronic Music System, or CEMS. It is unclear whether this was ever a catalog item, but apparently only one such unit was produced, from a commission by Joel Chadabe of the State University of New York - Albany. Its outstanding feature was a bank of four model 960 sequencers driven by a master clock source, under control of the performer.