### 3.1.1 Introduction to the LilyPond file structure

A basic example of a LilyPond input file is

>     \version "2.18.2"

>     \header { }

>     \score {
>        … compound music expression …   % all the music goes here!
>       \layout { }
>       \midi { }
>     }

There are many variations of this basic pattern, but this example serves as a useful starting place.

Up to this point none of the examples you have seen have used a `\score{}` command. This is because LilyPond automatically adds the extra commands which are needed when you give it simple input. LilyPond treats input like this:

>     \relative c'' {
>       c4 a d c
>     }

as shorthand for this:

>     \book {
>       \score {
>         \new Staff {
>           \new Voice {
>             \relative c'' {
>               c4 a b c
>             }
>           }
>         }
>         \layout { }
>       }
>     }

In other words, if the input contains a single music expression, LilyPond will interpret the file as though the music expression was wrapped up inside the commands shown above.

A word of warning! Many of the examples in the LilyPond documentation will omit the `\new Staff` and `\new Voice` commands, leaving them to be created implicitly. For simple examples this works well, but for more complex examples, especially when additional commands are used, the implicit creation of contexts can give surprising results, maybe creating extra unwanted staves. The way to create contexts explicitly is explained in Contexts and engravers.

Note: When entering more than a few lines of music it is advisable to always create staves and voices explicitly.

For now, though, let us return to the first example and examine the `\score` command, leaving the others to default.

A `\score` block must always contain exactly one music expression. Remember that a music expression could be anything from a single note to a huge compound expression like

>     {
>       \new StaffGroup <<
>          … insert the whole score of a Wagner opera in here …
>       >>
>     }

Since everything is inside `{ … }`, it counts as one music expression.

As we saw previously, the `\score` block can contain other things, such as

>     \score {
>       { c'4 a b c' }
>       \header { }
>       \layout { }
>       \midi { }
>     }

Note that these three commands – `\header`, `\layout` and `\midi` – are special: unlike many other commands which begin with a backward slash (`\`) they are not music expressions and are not part of any music expression. So they may be placed inside a `\score` block or outside it. In fact, these commands are commonly placed outside the `\score` block – for example, `\header` is often placed above the `\score` command, as the example at the beginning of this section shows.

Two more commands you have not previously seen are `\layout { }` and `\midi {}`. If these appear as shown they will cause LilyPond to produce a printed output and a MIDI output respectively. They are described fully in the Notation Reference – Score layout, and Creating MIDI files.

You may code multiple `\score` blocks. Each will be treated as a separate score, but they will be all combined into a single output file. A `\book` command is not necessary – one will be implicitly created. However, if you would like separate output files from one ‘`.ly`’ file then the `\book` command should be used to separate the different sections: each `\book` block will produce a separate output file.

In summary:

Every `\book` block creates a separate output file (e.g., a PDF file). If you haven’t explicitly added one, LilyPond wraps your entire input code in a `\book` block implicitly.

Every `\score` block is a separate chunk of music within a `\book` block.

Every `\layout` block affects the `\score` or `\book` block in which it appears – i.e., a `\layout` block inside a `\score` block affects only that `\score` block, but a `\layout` block outside of a `\score` block (and thus in a `\book` block, either explicitly or implicitly) will affect every `\score` in that `\book`.

For details see Multiple scores in a book.

Another great shorthand is the ability to define variables, as shown in Organizing pieces with variables. All the templates use this:

>     melody = \relative c' {
>       c4 a b c
>     }

>     \score {
>       \melody
>     }

When LilyPond looks at this file, it takes the value of `melody` (everything after the equals sign) and inserts it whenever it sees `\melody`. There’s nothing special about the name – it could be `melody`, `global`, `keyTime`, `pianorighthand`, or something else. Remember that you can use almost any name you like as long as it contains just alphabetic characters and is distinct from LilyPond command names. For more details, see Saving typing with variables and functions. The exact limitations on variable names are detailed in File structure.

#### See also

For a complete definition of the input format, see File structure.

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#### LilyPond — Learning Manual v2.18.2 (稳定分支).