18 bits was a common word size for smaller computers in the 1960s, when large computers often used 36 bit words and 6-bit character sets were the norm.
The UNIVAC produced several 18-bit computers, including the UNIVAC 418 and several military systems.
The IBM 7700 Data Acquisition System was announced by IBM on December 2, 1963.
The BCL Molecular was a group of systems designed and manufactured in the UK in the 1970s and 1980s.
The NASA Standard Spacecraft Computer NSSC-1 was developed as a standard component for the MultiMission Modular Spacecraft at the Goddard Space Flight Center (GSFC) in 1974.
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.Digital Equipment Corporation, PDP-9 Utility Programs--Advanced Software System--Programmer's Reference Manual, full text, Digital Equipment Corporation, 1968, Appendix 1.
The Teletype packs three characters in each 18-bit word; each character a 5-bit Baudot code and an upper-case bit.
The DEC SIXBIT format packs three characters in each 18-bit word, "PDP-7 Symbolic Assembler Programming Manual". p. 6; p. 38-39. each 6-bit character obtained by stripping the high bits from the 7-bit ASCII code, which folds lowercase to uppercase letters.