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Composers who don't happen to come from a percussion background often make certain common mistakes about how to use "roll" notation for percussion instruments.
Some common misconceptions:
What Roll Notation Says to a Percussionist
Adding those three slashes to a note's stem indicates a change from a percussive sound to a sustained one. This change is not only technical, in that the percussinist will use a special technique for creating a sustained sound, but also psychological, in that it informs the percussionist's interpretation and the kinds of things they'll do to make their playing musical.
Consider the following two completely different examples, for snare drum:
When playing the first, the percussionist will bounce their sticks in a special way to create as seamless and smooth a sound as possible. This is a very different sound from a stream of distinct attacks such as in the second example above. In orchestral literature, the first sound is very common and the second is rather rare. (However, streams of very fast non-rolled attacks have a place in situations where percussionists are expected to show off their technical facility, such as competitive solos, marching bands, and drum corps).
Ties and Slurs
The concert marimba is one of the few pitched orchestral instruments capable of playing both in a percussive style (like a piano) and in a sustained style (like a wind or string instrument); this flexibility can make the marimba a supremely versatile instrument if understood properly. When a roll is indicated, it tells the performer to think of their instrument as a wind or string instrument. Because of this, it is recommended to use all of the same marks for articulation one would expect to find in a wind part when writing rolls for mallet keyboard instruments.
A clarinet player (for example) will shape the beginning, middle and end of a note, no matter how short or long it is. To a clarinet player, the two examples below obviously indicate different performance practices:
This is because a clarinet player does more than simply blow air into the horn to create music: they carefully control the attack, body and release of each note. Likewise, a percussionist playing a roll does more than just hit the same bar on the marimba many times in a row. Although percussionists don't tongue their instruments (in public) and don't change bow directions, they are trained in a variety of subtle ways of controlling the mallets to create attacks and releases, and to swell, decay, or otherwise shape interior sections of notes in order to be musical. Because of all this, there is a difference in what a percussionist will do when they see each of these two examples:
Percussionists shape rolls musically even on less soft-spoken instruments. Even on a snare drum, ties should be used unless the composer intends each notehead to be rearticulated. On a snare, a percussionist can sneak into a roll from niente, sustain the roll as necessary, and decrescendo back into nothing. There is nothing "percussive" about that sound: the percussionist is creating a sustained sound instead. That's what a roll is.
A Roll is not a Tremolo
It should be clear by now that indicating a roll is very different from indicating an unmetered tremolo for a string instrument or a piano, even though they're both written with three slashes on the stem. A roll indicates a sustained sound, but strings (and to some extent the piano) already make a sustained sound by default; for these instruments the three slashes essentially add a texture to the sound. However, it is a mistake to assume that a roll will be 'textured' like an unmetered tremolo, especially because the percussionist will be doing everything they can to eliminate texture and create a smooth sound.
It's easy to see why percussionists can be a little insulted when a composer thinks that a roll means "hit the instrument as fast as possible with no particular meter." Percussionists know their playing to be more subtle, more nuanced, and more musical than this.
Rolling is so idiomatic and common for marimba that some composers will use a "Roll All Notes" indication, especially in solo literature, to save from having to write the three slashes on the stem of every note. This would be cancelled by "Roll Where Indicated," however, it is not rare to see the "Roll All Notes" indication applied to an entire piece or movement. The instrument supports this kind of writing just as well as it does the percussive kind.
A Word About Obsolete Notation Conventions
The percussion section has been explored by composers more thoroughly in the second half of the 20th Century than ever before, and the notation has become more modern and easier to read as a result. Be aware that older notation conventions for percussion instruments, such as the two for rolls in the following example, can still be found in all the well-known orchestration guides.
While familiarity with the above is useful for understanding older scores, these notational practices should be considered obsolete. The trill should never be used for a roll anymore, even on a snare drum where its meaning would be unambiguous. The use of disjointed tremolo notation for the keyboard instruments is unnecessary and irritating; please stem all of the notes together and use a single roll indication as shown in the earlier example.