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### 2.2.1 Music expressions explained

Analogy: mathematical expressions | ||

Simultaneous music expressions: multiple staves | ||

Simultaneous music expressions: single staff |

In LilyPond input files, music is represented by *music
expressions*. A single note is a music expression:

a'4

Enclosing a note in braces creates a *compound music
expression*. Here we have created a compound music expression
with two notes:

\relative { a'4 g4 }

Putting a group of music expressions (e.g. notes) in braces means that they are in sequence (i.e. each one follows the previous one). The result is another music expression:

\relative { { a'4 g } f4 g }

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#### Analogy: mathematical expressions

This mechanism is similar to mathematical formulas: a big formula is created by composing small formulas. Such formulas are called expressions, and they can contain other expressions, so you can make arbitrarily complex and large expressions. For example,

1 1 + 2 (1 + 2) * 3 ((1 + 2) * 3) / (4 * 5)

This is a sequence of expressions, where each expression is
contained in the next (larger) one. The simplest expressions are
numbers, and larger ones are made by combining expressions with
operators (like `+`

, `*`

and `/`

) and parentheses.
Like mathematical expressions, music expressions can be nested
arbitrarily deep, which is necessary for complex music like
polyphonic scores.

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#### Simultaneous music expressions: multiple staves

Music Glossary: polyphony.

This technique is useful for `polyphonic` music. To
enter music with more voices or more staves, we combine
expressions in parallel. To indicate that two voices should play
at the same time, simply enter a simultaneous combination of music
expressions. A ‘simultaneous’ music expression is formed by
enclosing expressions inside `<<`

and `>>`

. In the
following example, three sequences (all containing two separate
notes) are combined simultaneously:

<< \relative { a'2 g } \relative { f'2 e } \relative { d'2 b } >>

Note that we have indented each level of the input with a different amount of space. LilyPond does not care how much (or little) space there is at the beginning of a line, but indenting LilyPond code like this makes it much easier for humans to read.

**Note:** each note is relative to the previous note in the input,
only the first is relative to the `c''`

in the initial
`\relative`

command.

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#### Simultaneous music expressions: single staff

To determine the number of staves in a piece, LilyPond looks at the beginning of the first expression. If there is a single note, there is one staff; if there is a simultaneous expression, there is more than one staff. The following example shows a complex expression, but as it begins with a single note it will be set out on a single staff.

\relative { c''2 <<c e>> | << { e2 f } { c2 <<b d>> } >> | }

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