The GM 1 specification was superseded by General MIDI 2 in 1999 however, GM 1 is still commonly used. However, it still leaves the actual sounds of each instrument up to the supplier to implement one manufacturer's French horn, say, could be brighter, or more mellow, than another's. This helps ensure that playback of MIDI files sounds more consistently between different devices compliant with the GM specification. With MIDI 1.0, the assignment could be to an arbitrary instrument but with GM, a program number assigns a specific instrument name. For example, assigning one of the 128 possible MIDI Program Numbers selects an instrument.

Second, GM attaches specific interpretations to many parameters and control messages which were left unspecified in the MIDI 1.0 specification. First, GM requires that all compliant MIDI instruments meet a certain minimal set of features, such as being able to play at least 24 notes simultaneously ( polyphony). While MIDI 1.0 by itself provides a communications protocol which ensures that different instruments can interoperate at a fundamental level - for example, that pressing keys on a MIDI keyboard will cause an attached MIDI sound module to play musical notes - GM goes further in two ways. GM imposes several requirements beyond the more abstract MIDI 1.0 specification. The official specification is available in English from the MMA, bound together with the MIDI 1.0 specification, and in Japanese from the Association of Musical Electronic Industry (AMEI). GM was developed by the American MIDI Manufacturers Association (MMA) and the Japan MIDI Standards Committee (JMSC) and first published in 1991. General MIDI (also known as GM or GM 1) is a standardized specification for electronic musical instruments that respond to MIDI messages. For the British DJ, see General Midi (DJ). This article is about the electronic musical instrument specification.

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