X/Open Portability Guide - Computer Definition
An earlier name for standards that specified compliance with X/Open's Common Application Environment (CAE). XPG3 (1989) and XPG4 (1992) define operating systems, languages and protocols, etc. About half of the Single UNIX Specification (formerly Spec 1170) was taken from XPG4. The term "base" with XPG (XPG3 base, XPG4 base, etc.) referred to a minimum number of required APIs. See X/Open.