A notable proprietary fork not of this kind is the many varieties of proprietary [[Unix]]—almost all derived from AT&T Unix under license and all called "Unix", but increasingly mutually incompatible.<ref name=moen>[http://linuxmafia.com/faq/Licensing_and_Law/forking.html Fear of forking] {{webarchive|url=https://web.archive.org/web/20121217044712/http://linuxmafia.com/faq/Licensing_and_Law/forking.html |date=17 December 2012 }} - An essay about forking in [[free software]] projects, by Rick Moen</ref> ''See'' [[UNIX wars]].
The [[BSD licenses]] permit forks to become proprietary software, and some[[copyleft]] say{{who|date=Novemberproponents 2011}}say that commercial incentives thus make proprietisation almost inevitable. (Copyleft licenses can, however, be circumvented via dual-licensing with a proprietary grant in the form of a [[Contributor License Agreement]].) Examples include [[macOS]] (based on the proprietary [[NeXTSTEP]] and the open source [[FreeBSD]]), [[Cedega (software)|Cedega]] and [[CrossOver]] (proprietary forks of [[Wine (software)|Wine]], though CrossOver tracks Wine and contributes considerably), EnterpriseDB (a fork of [[PostgreSQL]], adding Oracle compatibility features<ref>[http://www.enterprisedb.com EnterpriseDB] {{webarchive|url=https://web.archive.org/web/20061113052033/http://www.enterprisedb.com/ |date=13 November 2006 }}</ref>), Supported PostgreSQL with their proprietary ESM storage system,<ref>[http://www.fastware.com.au/docs/FujitsuSupportedPostreSQLWhitePaperV1_02.pdf Fujitsu Supported PostgreSQL] {{webarchive|url=https://web.archive.org/web/20060820144738/http://fastware.com.au/docs/FujitsuSupportedPostreSQLWhitePaperV1_02.pdf |date=20 August 2006 }}</ref> and Netezza's<ref>[http://www.netezza.com/media/2004/Weblog.htm Netezza] {{webarchive|url=https://web.archive.org/web/20061113210650/http://www.netezza.com/media/2004/Weblog.htm |date=13 November 2006 }}</ref> proprietary highly scalable derivative of PostgreSQL. Some of these vendors contribute back changes to the community project, while some keep their changes as their own competitive advantages.
