Model M designates a group of computer keyboards designed and manufactured by IBM starting in 1985, and later by Lexmark International, Maxi Switch, and Unicomp. The keyboard's many variations have their own distinct characteristics, with the vast majority having a buckling-spring key design and swappable keycaps. Model M keyboards are notable among computer enthusiasts and frequent typists due to their durability, typing-feel consistency, and their tactile and auditory feedback. The popularity of the IBM PC and its successors made the Model M's design influential. Almost all later general-purpose computer keyboards mimicked its key layout and other aspects of its ergonomics. The layout was standardized by ISO in 1994 and ANSI in 1998, with minor additions—most notably the Windows key and Menu key. The Model M is regarded as a classic and durable piece of hardware. Although the computers and computer peripherals produced concurrently with them are considered obsolete, many Model M keyboards are still in use due to their physical durability and the continued validity of their ANSI 101-key and ISO 102-key layouts, through the use of a PS/2-female-to-USB-male adapter with a built-in level converter. Since their original popularity, new generations have discovered their unique functionality and aesthetics. It is estimated that during the IBM and Lexmark years, over 10 million Model Ms were shipped. Their mass-market success ended in the 1990s amid an industry-wide switchover to lower-cost dome-switch keyboards. IBM stopped producing the Model M keyboard in 1996. History The Model M keyboard was designed to be less expensive to produce than the Model F keyboard it replaced. Principal design work was done at IBM in 1983-1984, drawing on a wide range of user feedback, ergonomic studies, and examination of competing products. Its key layout, significantly different from the Model F's, owed much (including notably the inverted-T arrangement of its arrow keys) to the LK-201 keyboard shipped with the VT220 serial terminal. Production of Model M keyboards began in 1985. They were often bundled with new IBM computers. While today primarily associated with the IBM PC and its successors, it actually first shipped with the 3161 terminal and was deployed across several other IBM product lines as well, notably including the 5250 terminal and the RS/6000. They were produced at IBM plants in Lexington, Kentucky; Greenock, Scotland; and Guadalajara, Mexico. The most common variant is the IBM Enhanced Keyboard identified by IBM assembly part number 1391401, the U.S. English