### 3.4 Polyphony

The easiest way to enter fragments with more than one voice on a staff is to split chords using the separator `\\`. You can use it for small, short-lived voices or for single chords

>          \context Staff \relative c'' {
>            c4 << { f d e  } \\ { b c2 } >>
>            c4 << g' \\ b, \\  f' \\ d >>
>          }

The separator causes Voice contexts1 to be instantiated. They bear the names `"1"`, `"2"`, etc. In each of these contexts, vertical direction of slurs, stems, etc. is set appropriately.

This can also be done by instantiating Voice contexts by hand, and using `\voiceOne`, up to `\voiceFour` to assign a stem directions and horizontal shift for each part

>          \relative c''
>          \context Staff <<
>            \new Voice { \voiceOne cis2 b  }
>            \new Voice { \voiceThree b4 ais ~ ais4 gis4 }
>            \new Voice { \voiceTwo fis4~  fis4 f ~ f  } >>

The command `\oneVoice` will revert back to the normal setting.

Normally, note heads with a different number of dots are not merged, but when the object property `merge-differently-dotted` is set in the NoteCollision object, they are merged

>          \context Voice << {
>            g8 g8
>            \override Staff.NoteCollision
>              #'merge-differently-dotted = ##t
>            g8 g8
>          } \\ { g8.[ f16] g8.[ f16] } >>

Similarly, you can merge half note heads with eighth notes, by setting `merge-differently-headed`

>          \context Voice << {
>            c8 c4.
>            \override Staff.NoteCollision
>              #'merge-differently-headed = ##t
>          c8 c4. } \\ { c2 c2 } >>

LilyPond also vertically shifts rests that are opposite of a stem

>          \context Voice << c''4 \\  r4 >>

##### Predefined commands

`\oneVoice`, `\voiceOne`, `\voiceTwo`, `\voiceThree`, `\voiceFour`.

The following commands specify in what chords of the current voice should be shifted: the outer voice has `\shiftOff`, and the inner voices have `\shiftOn`, `\shiftOnn`, etc.

`\shiftOn`, `\shiftOnn`, `\shiftOnnn`, `\shiftOff`.

##### See also

Program reference: the objects responsible for resolving collisions are NoteCollision and RestCollision.

Examples: See also example files input/regression/collision-dots.ly, input/regression/collision-head-chords.ly, input/regression/collision-heads.ly, input/regression/collision-mesh.ly, and input/regression/collisions.ly.

##### Bugs

Resolving collisions is a intricate subject, and only a few situations are handled. When LilyPond cannot cope, the `force-hshift` property of the NoteColumn object and pitched rests can be used to override typesetting decisions.

When using `merge-differently-headed` with an upstem eighth or a shorter note, and a downstem half note, the eighth note gets the wrong offset.

There is no support for clusters where the same note occurs with different accidentals in the same chord. In this case, it is recommended to use enharmonic transcription, or to use special cluster notation (see Clusters).

#### Footnotes

[1] Polyphonic voices are sometimes called "layers" other notation packages

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