[See below for a discussion of the abc tune format.]
Another good way to find a tune, if you know its name, is to use a search engine (like Google) and search for the tune name followed by the word "abc". For example, if you are looking for The Rolling Wave, type in :
Most of these tune collection sites provide the tunes in several different file formats: ABC (see below); GIF, PDF (a graphic representation of the sheet music); and sometimes MIDI (a simple music file). If you find a tune only in abc format, and you would like to see the sheet music, the concertina.net site has an online conversion utility:
Simply copy the text version of a single abc tune into the box, and press "Submit". The utility resolves the abc file into sheet music which you can then print.
"abc" is a character notation for writing tunes in a portable and easy way, and is used for sending tunes via e-mail, for putting tune books on the internet, and for typesetting tunes. More information about the abc format is available at Chris Walshaw's
In addition to the description of the abc format itself, this site also has a huge list of other tune collections, as well as links to software designed to manage abc files.