In computer architecture, 18-bit integers, memory addresses, or other data units are those that are 18 bits (2.25 octets) wide.
Eighteen bits was a common word size for smaller computers in the 1960s, when large computers often using 36 bit words and 6-bit character sets, sometimes implemented as extensions of BCD, were the norm.
Possibly the most well-known 18-bit computer architectures are the PDP-1, PDP-4, PDP-7, PDP-9 and PDP-15 minicomputers produced by Digital Equipment Corporation from 1960 to 1975.
The flying-spot store digital memory in the first experimental electronic switching systems used nine plates of optical memory that were read and written two bits at a time, producing a word size of 18 bits.
The DEC Radix-50, called Radix 508 format, packs three characters plus two bits in each 18-bit word.