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It happens to the best of ensembles: In the middle of rehearsal, something will go wrong. Rather than start all the way back at the beginning of the piece — which would waste precious rehearsal time, not to mention result in rehearsing things that are already solid — the group will pick things up somewhere in the middle. To get everyone on the same page, they’ll often find one of these:
These are called rehearsal marks — in this format specifically, they’re rehearsal letters — and composers (or editors or engravers) will dot them thruout the score to mark significant landmarks in the piece to aid in the rehearsal process. In traditional Western musical notation, and still as the default setting for most engraving software today, they’ve taken the form pictured above: A capital letter inside a circle or a box. This is an unfortunate practice that should absolutely be discontinued — basically all rehearsal marks should be boxed bar numbers instead.
Obviously, rehearsal marks in general are an excellent idea. Big pieces of music are complicated, and having large, obvious signposts to orient yourself around has made many a rehearsal go much more smoothly than it otherwise would. But in solving the problem of “how do we begin together somewhere in the middle of a piece?”, using letters instead of bar numbers creates a number of unnecessary problems. Consider:
Missing or mis-placed letters are impossible to correct without consulting a score. Is rehearsal letter K in the right place in this excerpt?
Well, it seems plausible, but there isn’t really any way to know just from looking at the part (which, for a contemporary piece, may well be all the player has access to before the first rehearsal). If, instead of using letters, this piece were marked with bar numbers, then it would be obvious if the boxed 350 had somehow wound up in the wrong place. This is less common with computers than with hand engraving, but glitches happen. Saying “OK, everyone, let’s take it from letter K” is a lot less helpful when one person’s letter K is a bar off from everyone else’s — and in a complex, dissonant contemporary piece, it can be hard to tell that that’s what’s causing the issue instead of some player error or other.
The above excerpt also jumps directly from I to K. Many publishers do this to avoid risking confusion between I and J, but many other publishers don’t, and even I, notation-obsessed as I am, don’t have a ready list of which do which. Carefully checking the part beforehand — which, being honest, I’ve never actually done when practicing — might catch a missing C, D, or L (or however high the letters go), but a missing J would slip thru the cracks. Or at least, it would until the conductor asked to start there. Once again, there’s no way to tell where J should go just looking at that part (assuming it’s missing). If, on the other hand, bar numbers are used, I can still figure out how to start at bar 326 even if the big number in a box was inadvertently omitted from my part.
There’s no convenient way to add extra marks if the existing ones are too far apart. I still have a vivid memory of working on a piece by Johannes Brahms in youth orchestra and needing to add a rehearsal mark between rehearsal C and rehearsal D. (Brahms, for whatever reason, seems very keen on putting his rehearsal marks unusually far apart.) This particular edition didn’t print measure numbers at all (for reasons I cannot fathom, measure numbers in parts didn’t become standard practice until around the start of the 20th Century), so we wound up laboriously counting measures and adding “C1” somewhere between the two pre-existing marks. This was clunky enough, but then, of course, we wound up needing another mark between C and C1. We wound up using C0.5, but seriously, come on. With bar numbers instead of letters, you don’t have to deal with this unsightliness — you can add as many extra rehearsal marks as you need in the thick of rehearsal without having to resort to such ungainly constructions.
Letter marks in longer pieces can become confusing or even uncomfortable. Once you get past rehearsal letter Z, the tradition is to start doubling up: AA, BB, CC, and so on. After ZZ, triples start to happen. This is all well and good, until a rehearsal grinds to a halt because half the group thinks “Start at double U, please” means start at “W” while everyone else takes it to mean “UU” instead. (Not to mention the incredibly ungainly mouthful that is “rehearsal WW”.) Bartók Béla’s divertimento for string orchestra, meanwhile, runs on so long that he has to use rehearsal KKK, and no one wants that. Use bar numbers instead, and spare us the confusion and unintentional references to racial hate groups. (This is in addition to the fact that many letters sound alike. When someone’s rearranging percussion right behind you, it can be surprisingly difficult to tell whether the conductor wants you to start at B, C, D, E, T or V...)
Other Questionable Rehearsal Mark Life Choices
Rehearsal numbers. Instead of bar numbers, some publishers have made the inexplicable choice of putting numbers in boxes instead. So in the longer excerpt posted above, instead of replacing K with 350 in a box, they’d replace it with a boxed number 10. I suppose this marginally improves the odds of figuring out if one is missing and eliminates the UU/W ambiguity, but it creates a whole new danger: Confusing rehearsal numbers with bar numbers. Every single time I’ve played a piece with rehearsal numbers, this mistake has happened at least once. Every single time. Save yourself the trouble; use bar numbers.
Boxing every 5th or 10th bar number. This is definitely a step in the right direction, but it’s still not ideal. Remember that rehearsal marks are supposed to point out landmarks, significant musical events that you can use to orient yourself around. Especially when one of these marks lands one bar away from such a landmark, it’s very easy to slip up in your counting and think that the two coincide. The eight-bar phrases that make up the bulk of Western music also don’t play nicely with this system: Breaking up a standard eight-bar phrase into two bars of rest + five bars of rest + one bar of rest obscures the musical meaning with no added benefit. It’s easier to count a 32-bar rest as four tidy phrases than it is to keep track of an arbitrary and unmusical string of 5-bar rests that cover the same span. And just in general: Why would decide where to put your reference points without any regard for the places people are actually going to want to reference? Use bar numbers, but put them in places that make musical sense, don’t just strew them about algorithmically.
Times When Letters Make Sense
Having just written so many words inveighing against rehearsal letters, I have to grudgingly admit that there are times they make sense.
Pieces where different parts have different numbers of measures. I am often dubious of this as a notational practice, but sometimes people have decided that the best solution to a rhythmically intractable passage is to let each part have bars of different lengths, making no effort to synchronize the bar lines. See, for example, the last movement of Anton Webern’s second cantata:
Sure, some of the bar lines coincide, but rarely for more than a few measures in a row. In this circumstance, it’s not even clear exactly what bar numbers would mean. So if you’re in this boat, fine, use letters. (Tho it is worth pointing out that the letter C here will be floating ambiguously in the middle of the measure for the trumpet player; that part will need special handling so the player can tell which beat it falls on for them.)
Pieces that are likely to have large cuts or insertions mid-rehearsal. I’ve run into this most frequently with musicals, but I suppose it could happen for concert works too. If you know or suspect that you’re going to be adding or ditching chunks of music mid-rehearsal, having letters might be the way to go so that you don’t have to deal with saying “OK, this was measure 27, but now it’s measure 36, so change things accordingly”. Then again, in my experience musicals tend to preserve measure numbers at all costs; the most flagrant example I can think of is in one of the numbers from A Year with Frog and Toad, in which the measure numbers at one point go 22, 22a, 22b, etc all the way up to 22w, at which point they jump to 43 and continue on as expected. But once it’s done being re-written, I’m not sure how compelling the argument is to preserve this edit history between productions. Your mileage may vary.
Other than that, tho, boxed bar numbers are the way to go.