The NetHack General Public License is a copyleft

Copyleft

Copyleft is a play on the word copyright to describe the practice of using copyright law to offer the right to distribute copies and modified versions of a work and requiring that the same rights be preserved in modified versions of the work...

 software license primarily used by the roguelike

Roguelike

The roguelike is a sub-genre of role-playing video games, characterized by randomization for replayability, permanent death, and turn-based movement. Most roguelikes feature ASCII graphics, with newer ones increasingly offering tile-based graphics. Games are typically dungeon crawls, with many...

 game NetHack

NetHack

NetHack is a single-player roguelike video game originally released in 1987. It is a descendant of an earlier game called Hack , which is a descendant of Rogue...

. It is certified as an open source

Open source

The term open source describes practices in production and development that promote access to the end product's source materials. Some consider open source a philosophy, others consider it a pragmatic methodology...

 license by the Open Source Initiative

Open Source Initiative

The Open Source Initiative is an organization dedicated to promoting open source software.The organization was founded in February 1998, by Bruce Perens and Eric S. Raymond, prompted by Netscape Communications Corporation publishing the source code for its flagship Netscape Communicator product...

.

The license was written in 1989 by Mike Stephenson

Mike Stephenson

Michael "Mike" Stephenson, also commonly known as Stevo, is an English Rugby League World Cup winning Rugby League, and presenter/commentator on British satellite television channel Sky Sports who also represented Great Britain in the sport.Stevo, the nickname that he is commonly known as on TV...

, who patterned it after the GNU bison

GNU bison

GNU bison, commonly known as Bison, is a parser generator that is part of the GNU Project. Bison reads a specification of a context-free language, warns about any parsing ambiguities, and generates a parser which reads sequences of tokens and decides whether the sequence conforms to the syntax...

 license (which was written by Richard Stallman

Richard Stallman

Richard Matthew Stallman , often shortened to rms,"'Richard Stallman' is just my mundane name; you can call me 'rms'"|last= Stallman|first= Richard|date= N.D.|work=Richard Stallman's homepage...

 in 1988). Like the Bison license, and Stallman's later GNU General Public License

GNU General Public License

The GNU General Public License is the most widely used free software license, originally written by Richard Stallman for the GNU Project....

, the NetHack license was written to allow the free sharing and modification of the source code under its protection. At the same time, the license explicitly states that the source code is not covered by any warranty

Warranty

In business and legal transactions, a warranty is an assurance by one party to the other party that specific facts or conditions are true or will happen; the other party is permitted to rely on that assurance and seek some type of remedy if it is not true or followed.In real estate transactions, a...

, thus protecting the original authors from litigation.

Derivative works

The NetHack GPL requires all derivative works to be distributed under the same license (except that the creator of a derivative work is allowed to offer warranty protection on the new work). The derivative work is required to indicate the modifications made and the dates of changes. In addition, the source code of the derivative work must be made available, free of charge (except for nominal distribution fees).