.NIB File Extension
Interface Builder User Interface File
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What is a NIB file?
File created by Interface Builder, a software development program (included with Apple Xcode) used for building user interfaces for Mac OS X applications; saves a user interface component such as a window or a view as well as the text fields, buttons, and other interactive components.
While NIB files aren't required for Mac OS X applications, they are commonly used since creating interfaces visually in Interface Builder is often easier than creating them programmatically.
NOTE: Version 3 of Interface Builder introduced the .XIB format, which stores the same information as a NIB file but uses a text-based file format.