Reducing staff numbers in LilyPond

Question

I have a choral piece where some of it is 4-part divisi SATB, and some of it is unison (true unison, not octaves). I would love to notate this in LilyPond with the two (women's & men's) staves combining visually into one staff - when in the middle of a printed line, I'd like it to actually converge to the single staff, something like this (which I just pasted together crudely in an image editor):

Is that possible in LilyPond?

Answer 1

Short answer: yes, or pretty close.

Basic template:

top = {
  \repeat unfold 4 { b'4 }
}
mid = {
  \stopStaff s1 \startStaff
  g'4^\markup "Tutti" 4 4 4
}
low = {
  \clef bass
  \repeat unfold 4 { e4 }
}
words = \lyricmode {
  hel -- lo good -- bye
}
otherwords = \lyricmode {
  new staff is here
}
\score {
  <<
    \new ChoirStaff <<
      \new Staff \top
      \new Lyrics \words
      \new Voice = "new" <<
        \override Staff.TimeSignature.stencil = ##f
        \override Staff.Clef.stencil = ##f
        \override Staff.KeySignature.stencil = ##f
        \mid
      >>
      \new Lyrics \lyricsto "new" \otherwords
      \new Staff \low
    >>
  >>
}

I achieved the following given the image you provided. If I look further, there's probably a way to create that spanner bar between the top and bottom staves where there isn't one previously.

Long answer: it seems to me that if you're going for an unconventional manner to instruct all voices to sing in unison (and not octaves), then there are less strange ways of showing it. For instance, if the voices need to be in unison in the middle of the page, why not continue two staves until the line break, and then cut it down to one staff and write something like "all voices in unison"? That would seem to me to be less ambiguous.

Perhaps something like:

Answer 2

That's pretty awful. The usual way to typeset this is to keep the voices split while on the same line, then merge them. LilyPond has the VerticalAxis.remove-layer property for that sort of thing. Using it is sort-of complex. The regression test "divisi-staves" demonstrates how to do this.

If you indeed want to change in mid-line, \stopStaff and \startStaff are your friends. They should enable you to replace one staff setup with a different one.