The Moog company pioneered the commercial manufacture of modular voltage-controlled analog synthesizer systems. Company founder Robert Arthur Moog had begun manufacturing and selling vacuum-tube theremins in kit form while he was a student in the early 1950s and marketed his first transistorized theremin kits in 1961. Moog became interested in the design and construction of complex electronic music systems in the mid-1960s while completing a Ph.D. in Engineering Physics at Cornell University. The burgeoning interest in his designs enabled him to establish a small company (R. A. Moog Co., which became Moog Music and later, Moog Electronics) to manufacture and market the new devices.
Pioneering electronic music experimenters like Leon Theremin, Louis and Bebe Barron, Christopher R. Morgan, and Raymond Scott had built sound-generating devices and systems of varying complexity, and several large electronic synthesizers (e.g. the RCA Mark II Sound Synthesizer) had been built before the advent of the Moog, but these were essentially unique, custom-built devices or systems. Electronic music studios typically had many oscillators, filters and other devices to generate and manipulate electronic sound. In the case of the electronic score for the 1955 science fiction film Forbidden Planet, the Barrons had to design and build many circuits to produce particular sounds, and each could only perform a limited range of functions.
Early electronic music performance devices like the Theremin were also relatively limited in function. The classic Theremin, for example, produces only a simple sine wave tone, and the antennae that control the pitch and volume respond to small changes in the proximity of the operator's hands to the device, making it difficult to play accurately.
In the period from 1950 to the mid-1960s, studio musicians and composers were also heavily dependent on magnetic tape to realize their works. The limitations of existing electronic music components meant that in many cases each note or tone had to be recorded separately, with changes in pitch often achieved by speeding up or slowing down the tape, and then splicing or overdubbing the result into the master tape. These tape-recorded electronic works could be extremely laborious and time-consuming to create—according to the 1967 Moog 900 Series demonstration record, such recordings could have as many as eight edits per inch of tape. The key technological development that led to the creation of the Moog synthesizer was the invention of the transistor, which enabled researchers like Moog to build electronic music systems that were considerably smaller, cheaper, consumed far less power, and were far more reliable than earlier systems, which depended on the older vacuum tube technology.
1st commercially sold Moog synthesizer prototype
in 1964, commissioned by the Alwin Nikolais Dance Theater of NY 
Moog began to develop his synthesizer systems after he met educator and composer Herbert Deutsch at a conference in late 1963. Over the next year, with encouragement from Myron Hoffman of the University of Toronto, Moog and Deutsch developed the first modular voltage-controlled subtractive synthesizer. Through Hoffman, Moog was invited to demonstrate these prototype devices at the Audio Engineering Society convention in October 1964, where composer Alwin Nikolais saw them and immediately placed an order.
Moog's innovations were set out in his 1964 paper Voltage-Controlled Electronic Music Modules, presented at the AES conference in October 1964, where he also demonstrated his prototype synthesizer modules. There were two key features in Moog's new system: he analyzed and systematized the production of electronically generated sounds, breaking down the process into a number of basic functional blocks, which could be carried out by standardized modules. He proposed the use of a standardized scale of voltages for the electrical signals that controlled the various functions of these modules—the Moog oscillators and keyboard, for example, used a standard progression of 1 volt per octave for pitch control. This specific definition means that adding or subtracting control voltage simply transposes pitch, a very valuable feature.
At a time when digital circuits were still relatively costly and in an early stage of development, voltage control was a practical design choice. In the Moog topology, each voltage-controllable module has one or more inputs that accept a voltage of typically 10 V or less. The magnitude of this voltage controls one or more key parameters of the module's circuits, such as the frequency of an audio oscillator (or sub-audio "low frequency" oscillator), the attenuation or gain of an amplifier, or the cutoff frequency of a wide-frequency-range filter. Thus, frequency determines pitch, attenuation determines instantaneous loudness (as well as silence between notes), and cutoff frequency determines relative timbre.
Voltage control in analog music synthesizers is similar in principle to how voltage is used in electronic analog computers, in which voltage is a scaled analog of a quantity that is part of the computation. For instance, control voltages can be added or subtracted in a circuit almost identical to an adder in such a computer. Inside a synthesizer VCO, an analog exponential function provides the 1 volt per octave control of an oscillator that basically runs on a volts/kHz basis. Positive voltage polarity raises pitch, and negative lowers it. The result is that, for example, a standard keyboard can have its output scaled to that of a quarter-tone keyboard by changing its output to one-half volt per octave, with no other technical changes.
Using this approach, Moog built a range of signal-generating, signal-modifying and controller modules, each of which could be easily inter-connected to control or modify the functions and outputs of any other. The central component was the voltage-controlled oscillator, which generated the primary sound signal, capable of producing a variety of waveforms including sawtooth, square and sine waves. The output from the VCO could then be modified and shaped by feeding the signal into other modules such as voltage-controlled amplifiers, voltage-controlled filters, envelope generators, and ring modulators. Another customization as part of the Moog Modular Synthesizer is the sequencer, which provided a source of timed step control voltages that were programmed to create repetitive note patterns, without using the keyboard. The inputs and outputs of any module could be cross-linked with patch cords (using tip-sleeve ("mono") ¼-inch plugs) and, together with the module control knobs and switches, could create a nearly infinite variety of sounds and effects.
The final output could be controlled by an organ-style keyboard as the primary user interface, but the notes—individual sounds—could also be triggered and/or modulated by a ribbon controller or by other modules such as white noise generators or low-frequency oscillators. The Moog modular systems were not designed as performance instruments, but were intended as sophisticated, studio-based professional audio systems that could be used as a musical instrument for creating and recording electronic music in the studio.
1CA Chris Swanson Modular System in 1965, is an earliest system for demonstration.
Moog's first customized modular systems were built during 1965 and demonstrated at a summer workshop at Moog's Trumansburg, New York, factory in August 1965, culminating with an afternoon concert of electronic music and musique concrète on August 28. Although far more compact than previous tube-based systems (e.g. the RCA Mark II) the Moog modular systems were quite large by modern standards, since they predated the introduction of integrated circuit ("microchip") technology; one of the biggest of these, the Moog-based "TONTO" system (built by Malcolm Cecil and used by Stevie Wonder in the 1970s) occupies several cubic meters when fully assembled. These early Moogs were also complex to operate—it sometimes took hours to set up the machine for a new sound—and they were prone to pitch instability because the oscillators tended to drift out of tune as the device heated up. As a result, ownership and use was at first mainly limited to clients such as educational institutions and major recording studios and a handful of adventurous audio professionals.
Ca. 1967, through contacts at the Columbia-Princeton Center, Moog met Wendy Carlos, a recording engineer at New York's studio Gotham Recording and a former student of Vladimir Ussachevsky. Carlos was then building an electronic music system and began ordering Moog modules. Moog credits Carlos with making many suggestions and improvements to his systems. During 1967 Moog introduced its first production model, the 900 series, which was promoted with a free demonstration record composed, realized and produced by Carlos. After assembling a Moog system and a custom-built eight-track recorder in early 1968, Carlos and collaborator Rachel Elkind (secretary to CBS Records president Goddard Lieberson) began recording pieces by Bach that Carlos played entirely on the new Moog. When Moog played one of their pieces at the AES convention in 1968 it received a standing ovation.
The use of flexible cords with plugs at their ends and sockets (jacks) to make temporary connections dates back to cord-type manually operated telephone switchboards (if not even earlier, possibly for telegraph circuits). Cords with plugs at both ends had been used for many decades before the advent of Moog's synthesizers to make temporary connections (patches) in such places as radio and recording studios. These became known as patch cords, and that term was also used for Moog modular systems. As familiarity developed, a given setup of the synthesizer (both cord connections and knob settings) came to be referred to as a patch, and the term has persisted, applying to systems that do not use patch cords.