Many musical acts on board a ship -- from piano bar entertainers to party bands to cocktail pianists -- learn the songs that make up their respective repertoires either purely by ear, or with written music that they have time to look at and study beforehand. Orchestra musicians bear the unique responsibility of having to sight-read all of their music upon their arrival to a new ship, and the music they read varies greatly in terms of content. Some charts may consist entirely of written-out notes. However, far more common is a combination of notes and slashes -- commonly referred to as a 'slash chart' -- a means of conveying the music which all orchestra members must become comfortable reading and playing.
Slash chart interpretation is a skill crucial to becoming a successful orchestra musician. A slash chart simply refers to any musician's written part where the majority of the music is conveyed through back slashes instead of actual notes. This technique of part-writing is most commonly used for rhythm section instruments (guitar, keys, bass, drums); horn players usually only see slashes for soloing purposes, and so this topic of discussion will be less relevant to those players.
Slash charts vary heavily in terms of their specificity. A guitar chart may convey a certain strumming pattern along with a chord or multiple chords, followed by those same chords with four simple back slashes (in a 4/4 time signature, of course). A piano chart may include note heads and chords and then the same back slashes, a drum chart may write out a two-bar beat and then the slashes, and so on. Many charts will include rhythmic hits throughout the piece of music for drums, and those same hits coupled with chords for bass, keys and guitar. Conversely, some music won't even convey a rhythmic pattern at the beginning, and will simply show harmonies for chordal instruments or attach a style heading for a drummer. Orchestra musicians must be prepared to deal with any type of slash chart (although, obviously, the more information in a chart, the better).
The two keys to interpreting a slash chart well are simply using one's ear and a having a strong grasp of many musical styles. If a keyboardist is reading a very vague slash chart without a listed style at the top, but hears, as soon as the band begins the song, that the style is disco, he should assume that he'll likely be playing held-out chords with a rhodes or a strings patch. A guitarist with a similarly-vague chart in a funk song should assume he'll be playing a heavily rhythmic strumming pattern, raking strings and possibly using a wah pedal. A bassist in a Merengue setting should know to play a quarter-note pattern and turn his treble way down. And a drummer on a Motown chart should know that he needs to drive the song but not overplay. The faster a musician can make a snap judgment on the genre of music and the role that his instrument plays in that style, the more positive an influence he'll have on the band as a whole.
And finally, a slash chart should, essentially, tell a musician that he has some homework to take care of after the set. Unquestionably, the best way to learn to play a song is by listening to a recording of that song. Any vague or unclear aspects of a slash chart will immediately be erased; guitarists and keyboardists can hear exactly what patches are being used, drummers and bassists can lock onto the exact style, as well as the specific patterns, fills, and so on. Some small amount of transcribing may be necessary, but it will result in a much more accurate interpretation of the song. And as important as it is to sight-read a slash chart well, it's just as imperative -- if not more so -- to nail that chart the second time.