...Hewlett-Packard - have fairly equal shares of the market. All three versions of Unix conform to the standards set by the OpenGroup's Single Unix Specification and can run on other "qualified" hardware, though not necessarily with the full range...
http://www.computerweekly.com/Articles/2007/12/18/228653/hot-skills-aix-6.1.htm
...IBM with AIX, and Hewlett-Packard with HP-UX. All three conform to the single Unix standard maintained by the OpenGroup, and can run on one another's hardware. All three jockey to introduce technical innovations which are quickly taken...
http://www.computerweekly.com/Articles/2008/01/08/228816/hot-skills-hp-ux.htm