ABC syntax - How to write ABC

Contents - How ABC works

Note Pitches
Note Length
Bridging
Bar Lines and Repeats
About Fields
The X: Field
The T: Title Field
The M: Metre Field
The L: Field
The Q: Tempo Field
The K: Key Signature Field
A Minimal Tune
Accidentals
Information Fields
Something Chromatic
Extending ABC

The pitch of each note is defined by the letter of its common name, the lower case being used to signify the upper of two octaves, thus:
CDEFGABcdefgab.
To extend the range downward, we append a comma, thus:
C,D,E,F,G,A,B,CDEFGABcdefgab.
To extend upward, we append an apostrophe (ASCII code 0180), sometimes called "prime", thus: CDEFGABcdefgabc'd'e'f' and so on. This is further extensible by doubling (or even tripling) the commas and primes. And before you ask, the answer is No; we don't go further than that because by then we've accounted for all the white notes on a piano. I'll deal with the black notes later, under accidentals.

Note Length
Anyone who has spent any time listening to music will have realised that some notes last longer than others. Most early music notation simply showed notes as either long or short, and the long note typically had a duration twice that of the shorter. As history unfolded, even shorter notes were demanded, and assumed to be half the length of the previous short note. This process of dividing by two gives us our standard musical symbols for our printed notes, and some their names.
ABC allows for this by letting you define a note length as the "default" note length, then every note in the tune is assumed to be that length, unless you indicate otherwise. The American method of naming note lengths is adopted; so English users need to remember that a "quarter-note" is a crotchet. (I help myself to remember this way - I can always remember that a crotchet is either a 1/4 or 1/8th note, but recall that a quaver is NOT a quarter note.)
The default note length in ABC is given by the "L field" which should be placed before any information on the notes in the tune. Thus
L:1/8
indicates a default note-length of a quaver, so the notes Bcd would indicate three quavers rising from B to d.
To indicate a note twice as long as the default (with L:1/8, that would mean a crotchet), place a number 2 after the pitch letter, so A,2B,2C4 would indicate two crotchets rising to a minim on middle C.
Sub-divisions of the default note length are denoted by placing an "oblique" (also known as a "slant" or "forward slash") after the pitch letter. Thus, G/A/B stands for two semi-quavers rising to a quaver on B above middle C. It can be written as G/2A/2B but the 2s don't add anything to the readability. On the other hand, if you need an occasional pair of demi-semi-quavers, use something like G/4A/4.
We're going to use the full-stop (American "period") later to signify staccato playing of the notes, so to indicate that a note is one-and-a-half times the default length, use a multiplier of 3/2. For example - G3/2G/2 or G3/G/ both indicate a dotted pair of quavers.
This notation also has a shorthand version - G>G.

Bridging

Quite simply, if it is legal for notes to be bridged (as in G>G above), then the bridge will be drawn unless you leave a space between them. Here's G> G

Bar Lines and Repeats

Well, it's nice to know some things are simple. On your computer keyboard there is a key with the "backslash" symbol, and the same key is used to enter the bar symbol - just hold down one of the shift keys first. I don't know why, but the keys on most keyboards show this symbol with a break in it, halfway up. Predictably, a |: is the start, and :| the end of a repeated section. First- and second-time sections are shown by (main section here) |1 (first-time section here) :|2 (second-time section here) ||. You can show a doubled bar line like that, and a heavy double bar line with |].

About Fields

There is some information that the ABC standard demands is included in every tune. It must appear directly above the text which represents the notes of a tune. ABC needs to know a reference number for the tune (the X: field), a title for the tune (the T: field) and the text of the tune. ABC assumes you have finished a tune when you leave a blank line. I recommend that you also include a metre field (M:), a default note-length field (L:), a tempo field (Q:) and a key signature (K:) field as a working minimum. Each field must start in the left-most column.

The X: Field

It doesn't seem to matter much which number you put after the X: but some programs find them useful for finding particular tunes in a file, and can get confused if several tunes have the same X: number.

The T: Title Field

Surprise, surprise! Here is where you put the tune's title. You can put several T: fields into each tune, though some ABC software ignores all but the first.

The M: Metre Field

Here you put the time signature of a tune. So a waltz would include M:3/4, a jig M:6/8 and so on.

The L: Field

Here you put the default note-length (in terms of which the tune is written). It does not have to agree with the lower of the two time-signature figures, but if it is absent, most ABC interpreters take a hint from the M: field.

The Q: Tempo Field

I advise using the following form - Q:1/4=180 which tells ABC software to play the tune at 180 crotchets per minute. This usually does for reels, 120 for waltzes. Jigs I usually give as Q:3/8=120, polkas and marches I adjust according to the mood.

The K: Key Signature Field

You can put C for C major, Cm for C minor, and even the modes are catered for: Cdor, Clyd, Cphr, Cmix, Caeol for dorian, lydian, phrygian, mixolydian and aeolian. Or you can write modes in full - Gmajor, Aminor, Gdorian etc.

A Minimal Tune

X:1
T:A Scale of G
M:4/4
L:1/8
Q:1/4=140
K:G
|:G2 AB|cd ef|g2 d2|B2 G2:||
a simple scale in G Note that the ABC takes care of sharpening the f automatically. You can download an MP3 file of this to listen to.
The picture of the music is generated from the ABC file.

Accidentals

Sharps and flats may be added to individual notes by prefacing them with a caret ^ for a sharp, an underscore _ for a flat or an equality = for a natural. Double sharps ^^ and double flats __ are also catered for.

Note that a line of ABC text does not necessarily equate to a line of stave: the continuation character is \ (oblique-left or "back-slash").
You can download an MP3 of this or, if you have a MIDI-enabled player, a midi file of the above fragment.

Information Fields

We can also add information about a tune: we use C: for the composer, S: for the source of the tune (e.g. who you learned it from), O: for the country of origin, B: for the books that contain the tune, D: for a discography of recorded versions, H: for historical notes, N: for general notes, and Z: for the person who transcribed the tune into ABC. This is not an exhaustive list; there is even provision for user-declared fields.

Extending ABC

Since its first incarnation, ABC has been extended to enable it to encompass multi-part tunes (even orchestral scores), and to embed MIDI instructions which allow for the selection of different instrument sounds, and include expression marks and other dynamics. Lyrics are also catered for, guitar chords, tablature notations, transposing instruments and a host of other features. The most useful discussion list I have found is abcusers, which is hosted by Yahoo! groups.