I'd like to clarify the use of some terms: When I want to name just one "line" of music if there's just one instrument written, what would be the correct word? Do I say "the second bar of the third system?", "the second bar of the third staff?" etc ... Would this name still apply even in the case of a single guitar tablature staff? Staff means the whole set of all lines in the page? What about the case of multiple instruments and I want to point some specific bar? Would it be something like "the first bar of the second staff of the third system"? I know in practice nobody would speak like this, but I just want to have the right definition of this terms.
So, how to refer to a line of music in a score or part in English?
If we're talking about a single instrument that uses a single staff (or stave), e.g. a sheet of music for guitar, either notes or tablature, one could say:
- bar (or measure) X (e.g. bar 12 — i.e. from the beginning of the piece);
- bar X of line Y (e.g. bar 4 of line 3);
- bar X of the Yth staff (e.g. bar 4 of the 3rd staff).
This would apply to a solo piece, or a part score from an ensemble piece.
With an instrument that uses a multiple staves, like piano or harp, one could refer to lines or systems to refer to the group of staves. So, for a piece for piano one could refer to "bar 4 of line 3" or "bar 4 of system 3". To refer to a specific bar in a specific staff of this piano part, you could refer to the staff by
- function — right hand, left hand —,
- clef — treble, bass —,
- position — top, bottom (or middle if there are more than 2) —,
whatever is clearest in context. So you might refer to "the left hand in bar 4 of line 3". (Organ parts add another layer of complexity here, but organists are super-humans who are used to translating from our mundane language.)
When we get to ensemble scores, each group of staves is a system. Each instrument in a system is a part, or staff. It's easiest to refer to the individual staves by their instrumental part name: e.g. flute, 2nd trombone, timpani, harp, chorus, double-basses. In most scores the instrument names are marked down the left margin, so one can easily refer to "3rd system, 4th bar, 3rd horn" or "viola in the 4th bar of the 3rd system". There are older scores (especially from the Soviet era) that only mark changes in the instruments presented from system to system, so one may have to describe a bar thus: "bar 4 of the 2nd staff in the 3rd system".
[added] As Tim points out in the comments below, these rules don’t apply if you’re trying to navigate a score with an ensemble. Parts for the individual players will be laid out differently from the full score (except for choral scores, but more on that later), so the best way to communicate a location to the players is bar numbers & rehearsal marks.
With choral music, at least small scale works for choir with keyboard accompaniment or for a cappella works, everyone — singers, conductor, accompanist — will work off the same score, so one is able to say “let’s go from the bass entry at the upbeat to bar 4 of the 3rd system” & everyone will be able to home in on the same point in the music. One couldn’t use such an orientation phrase in a band or orchestra just because the parts for each player are differently laid out from each other & from the full score.