XPG4

X/Open Company, Ltd., originally the Open Group for Unix Systems, was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of information technology. More specifically, the original aim was to define a single specification for operating systems derived from UNIX, to increase the interoperability of applications and reduce the cost of porting software. Its original members were Bull, ICL, Siemens, Olivetti, and Nixdorf—a group sometimes referred to as BISON. Philips and Ericsson joined soon afterwards, at which point the name X/Open was adopted.

The group published its specifications under the name X/Open Portability Guide (or XPG). Issue 1 covered basic operating system interfaces, and was published within a year of the group's formation. Issue 2 followed in 1987, and extended the coverage to include Internationalization, Terminal Interfaces, Inter-Process Communication, and the programming languages C, COBOL, FORTRAN, and Pascal, as well as data access interfaces for SQL and ISAM. In many cases these were profiles of existing international standards.

XPG3 followed in 1988, its primary focus being convergence with the POSIX operating system specifications. This was probably the most widely used and influential deliverable of the X/Open organisation.

By 1990 the group had expanded to 21 members: in addition to the original five, Philips and Nokia from Europe; AT&T Corporation, Digital, Unisys, Hewlett-Packard, IBM, NCR, Sun Microsystems, Prime Computer, Apollo Computer from North America; Fujitsu, Hitachi, and NEC from Japan; plus the Open Software Foundation and Unix International.

...
Wikipedia