Line continuation (\) should be used sparingly, and never to join more notes into a line than can comfortably fit on a single staff on the printed page (i.e. about 7 inches). Some programs choke on line continuations, and they are fraught with other problems. One of the biggest problems with ABC is that such features have been inconsistently implemented by various programs, rendering them useless or at least risky in the wider domain.

Repeats should always be bracketted at front and end, and (despite older usage) should not be placed anywhere but at the beginning or end of full bars. This doesn't accord with how repeats actually align in songs and in dance music, and necessitates the use of first and second repeats, but taking the more straightforward, economical approach will cause many programs to report errors or choke. Also note that ":||:" at the end of a tune is an error, since there is nothing to repeat on the right-hand side.

Bars should not be split across lines, regardless of how natural such a division may be logically. The pickup notes should be placed on the preceding line, within the bar to which they belong.

There is a notable exception: section markers (double bars) may be placed mid-bar. When this happens, the following pickup notes may be placed on the following line, as at the beginning of a tune. But any repeats associated with the section must still fall on full bar lines.

The last measure of a piece should be rest-padded to form a complete measure. This should be done even if, in practice, no rest occurs there (for instance, because the tune typically recycles to the beginning, with the last measure being completed by the initial pickup notes). It is understood that the rests serve as place-holders for the pickup notes except when the tune finishes.

Source information should be placed in S: directives; these are more likely to be printed than N: directives, and when both are printed, source information should precede the notes. Source and notes are usually labelled as such.

Finally, the tempo marking (Q directive) should be specified in terms of the primary beat duration. For 4/4 or 3/4, this would be 1/4=xx; for cut time (2/2 or C|), this would be 1/2. 6/8, though technically meaning six beats per measure, usually means either two or three primary beats per measure, with the beat being either the duration of a quarter note or a dotted quarter note, hence the tempo should be expressed as 1/4=xx or 3/8=xx (even if the tune switches between duple and triple groupings of eighths). When a measure doesn't evenly subdivide into constant primary beats (like 7/8 or 11/16), you're best off treating the entire measure as a single beat unit or specifying the most common primary beat size (like 1/4). If the tempo marking gets into the 200 range or beyond, you've likely chosen the wrong unit--just listen to a metronome clicking at this rate and you'll see why.

So, with these tips in mind, here's "Malltraeth" more properly (or portably) rendered into ABC:

  X: 1
  T: Malltraeth
  C: Welsh traditional
  S: from "Musical and poetical relicks", Edward Jones
  N: Malltraeth, pentre yn ne orllewin Sir Fon
  N: Malltraeth, a village in south west Anglesey
  M: 3/4
  L: 1/8
  Q: 1/4=100
  K: G
  dc |: B2 GBAG|F2 DFEC|B,2 D2 GF|G4 dc|
  B2 g2 f2|e2 A2 d2|^c3 d e^c |1 d4 dc :|2 d4 ||
  d2 |: edefge|d2 B2 cd|e2 EFGA|G2 FEDC|
  B,2 D2 GF|E2 c2 B2|A3/2c1/2 D2 GF |1 G4 d2 :|2 G4 z2 |]

For future reference, be careful when posting ABC tunes with embedded angle brackets. To ensure the HTML parser doesn't get confused and display your ABC improperly, you should translate these to "&gt;" (>) and "&lt;" (<)--and always preview, because when it comes to the entry box, what you see is not always what you get.