ABC notation was invented by English bagpiper and computer scientist Chris Walshaw as a way of writing down a quick ‘tune skeleton’ using letters, numbers and punctuation. The notation style means that players can send each other traditional tunes via email, recording the ‘essentials’ of the tune rather than the details of every ornament and variation — in line with the idea that traditional tunes are rough templates, rather than precise compositions.
The definitive guide to abc notation can be found at Chris‘s abc home page.
ABC is designed to be easy to learn and use, rather than functioning as a complete notation system. At its simplest, it can appear as a list of note names, with a few numbers to show notes of different lengths. For example, the first 4 bars of The Road to Skye could be written as:
GGG FGD | GBd e2f| gBB {c}BAB | cBc E2D |
in abc notation, which is equivalent to 
in conventional notation.
What makes abc notation different from both conventional notation and computer music languages such as MIDI is that it is designed to be both human-readable and machine-readable. There are many software applications that can read abc notation and either convert it into conventional notation or play it through the speakers of your computer. This means that abc can even be used by musicians who prefer to learn tunes by ear — with the added advantage that the tune can be repeated indefinitely, and at any speed.
Some programs (such as BarFly) can even add standard ornaments and rhythmic feel to the tune as it is played, giving a reasonable imitation of a traditional player playing the tune.
There are thousands of tunes from a wide range of regional traditions available on the web, and even search engines where you can look for a sequence of notes. This can be handy if you can only remember a brief fragment of a tune but can‘t recall the name!
