Inform is an authoring system for Interactive Fiction. It is written and maintained by Graham Nelson, and was first introduced in 1993. It can compile to the Z-code story file format, and newer versions can also compile to the Glulx file format. Inform source code files are standard text files. In versions prior to Inform 7, the language is a traditional programming language. In Inform 7, a new "natural language" was introduced, allowing games to be written using simple English sentences.
- .inf: the standard extension for an Inform source code file in a version earlier than Inform 7.
- .i7: The standard file extension for an Inform 7 source file.
- .i6: Sometimes seen to refer to Inform 6 files as opposed to Inform 7 files, usually in projects mixing both Inform 7 and Inform 6 code, and intermediate files generated by the Inform 7 natural language parser.
- .txt: When Inform 7 is instructed to include the source in the game publishing package, it puts the source code in a plain text file.
- Balances.inf: Balances (source code), by Graham Nelson
- vespers.inf: Vespers (source code), by Jason Devlin
- bronze/source.txt: Bronze (source code), by Emily Short
- FragileShells.txt (source code), by Stephen Granade