This video, via Synthfool, captures a Buchla Series 100 modular synthesizer in action.

Per Buchla:

The Modular Electronic Music System is composed of functional modules, each designed to generate a particular class of signals or perform a specific type of signal processing. Each module is 7 inches high and 4 1/4 inches (or an integral multiple thereof) wide. Up to 25 modules sharing a single power supply may be assembled in a single cabinet to form a super-module.

The system employs three varieties of signals, each with a distinctly different function:

Audio signals, the raw material of electronic music, are formed by various sorts of generators (sine, square, sawtooth, harmonic) or are produced externally (tape loop, radio, microphone). In composing a piece, signals may be filtered, gated, mixed, modulated, or otherwise processed. A standard level of 0 dB (ref. 600 Ohms) is employed for audio signals within the system.

Control voltages, used to determine frequencies, envelope characteristics, amplitudes and other parameters, are generated by keyboards, programmable voltage sources and envelope generators. The standard control voltage range is from 0 to 15 volts.

Timing pulses are originated by keyboards, programmable sequencers, and pulse generators. They are used to trigger notes, open gates, or initiate chains of musical events. Timing pulses are about 15 volts in amplitude.

The rules for interconnecting are straight-forward. Any number of inputs may be connected to a single output. Timing pulse outputs may be paralleled and connected to one input. The system output may be derived from any module; output is of sufficient magnitude to drive line inputs on tape recorders or sensitive inputs on power amplifiers.