There is only one file type that uses the .LY extension:
Input file for LilyPond, a musical score creation program; contains header information, such as the title and composer, followed by one or more musical scores; each score includes the clef, time, and key, followed by a list of notes and note durations.
LY files are saved in a text format that can be edited with a text editor. They can be processed by LilyPond and turned into printable compositions.
Updated: July 4, 2009