I was in a rehearsal recently when a colleague asked me start on the note G. Figuring that we would be starting in a place that we had just been working on I played a particular passage that turned out to be the wrong on. My colleague grew irritated, began raising their voice and rushed to my side while telling me to "start at the G!" Unfortunately for me, both pages that I was open to were littered with G's. This would be like going to the produce aisle and telling someone to pick up "the banana" rather than "a banana". The frustration grew for both of us until this person pointed out where on the page I was to play. In the end my colleague was visibly angry, I was simmering (a polite Minnesotan type of anger) and everyone else in the room was uncomfortable.
This entire situation could have been easily remedied if either A) I could read minds; or B) my colleague had used the proper language to express what they wanted. Situations like this, where we need to find a place in music, happen dozens of times every day (sometimes hundreds) for a professional musician. Luckily, thanks to centuries of musical dialogue we have a system in place to assist in this exchange of information. Below is a guide to talk about musical location from the general to the specific.
Page numbers are useful in some situations, but are not always a reliable way to talk about musical location. For example, if everyone in a choir has the same edition of a given piece, say Poulenc's Secheresses, than they can all easily turn to the same page. If they are working out different editions this may not work. Different editions may fit differently onto the page for any variety of reasons and page numbers simply will not match up. The third movement of one edition may be on page 35 while another starts it on page 42.
Additionally, page numbers will only work for people working out of the same instrument part. Two piano parts from the same publisher will probably have the same pages, but the violin part will not. This is because the violin part can all be fit onto a smaller number of pages than the piano score. This is why we have measure numbers and rehearsal letters.
Measure Numbers and Rehearsal Letters
Measure numbers are the easiest way for musicians to communicate their location in a score. In this system every measure of music (the space between two of those vertical lines that show up every inch or two on the page) is given a sequential number. Sometimes an editor will place a measure number before the first measure of every system (more on systems below). By telling a group of people, or any other musician working on the same piece which measure to start on, everyone can easily find their way regardless of which instrument they are playing or what edition they have, assuming that everyone is playing the same work and that the editions don't vary widely or have missing measures.
Sometimes a longer piece with a lot of measures will give occasional rehearsal letters in lieu of measure numbers. These are usually placed in a box above the staff and show up at every major event in the piece. These are useful because you don't have to say out loud large numbers that get lost, and the letters are usually placed at strategic points that make it easy to guide a rehearsal. Their great disadvantage is a lack of specificity.
If a piece only has rehearsal letters, they must be used in conjunction with the techniques used to below to be really specific.
In a piece that lacks measure numbers (and rehearsal letters), it is necessary to name which system of music you want to work on. A system is a group of staves that have been grouped together. So in a choral piece, it will usually be the soprano, alto, tenor and bass lines as well as the piano. Here is an example:
In the above picture, the three string instruments are all tied together with a bracket. This bracket tells a musician that if they finish one line of music that they should move to the next bracket instead of just moving down to the next staff. Here each staff has its own clef. So the violin gets its own staff as does the viola and cello, but together they make up a system. Therefore it is erroneous to call the system a staff. To refer to the "first system" as the "first staff" beside confusing the singular with the plural (assuming the speaker actually means the first system, she should be using the plural "staves" to refer to all of them), this would really mean only the very top staff. In the above case it would refer only to the violin.
If you want to communicate to a musician where to begin in music that only has page numbers, one only has to give the page number followed by which system, and then follow that up with the steps below.
Measures and Beats
In the situation above, where there aren't measure numbers, the musician may have to specify within the system
We can be specific with beats regardless of how much information is provided to us on the page. We can say "measure 248, beat 3" or we can say "page 5, system 3, measure 2, beat 2". We can also count measures in relation to other landmarks. For example, "five measures before A" or "two measures after GG".
Although it may seem like a mouthful and a lot of talking to do something simple, this language has evolved and stuck with us for a reason. It is the most efficient means of talking about music. It lets us all share a common thread on which to build and helps us work together. If we can all sign on to this system of language we can avoid disputes like the one above, and keep our music-making meaningful instead of frustrating.