APA (7th ed.) Citation

X/Open Group. (1985). X/OPEN portability guide. Elsevier Science Publishers B.V..

Chicago Style (17th ed.) Citation

X/Open Group. X/OPEN Portability Guide. Amsterdam: Elsevier Science Publishers B.V., 1985.

MLA (9th ed.) Citation

X/Open Group. X/OPEN Portability Guide. Elsevier Science Publishers B.V., 1985.