How to move placement of key signature in Lilypond

Question

In LilyPond (ver. 2.18.2), I'm notating some Ligeti (as a challenge, mostly). When writing out a snippet of his Étude En Suspens, mm. 18-21, there's a passage with different keys between the bottom and top staff (bottom in C major at bar 18, top in D♭ major). Three measures later, they swap keys (bottom in D♭, top in C).

Question: How can I get LilyPond to keep both staves' key signatures aligned?

Screenshot of PDF created by LilyPond:

LilyPond Code

\version "2.18.2"
\include "english.ly"
\include "articulate.ly"

\pointAndClickOff
#(set-global-staff-size 20)

\header
{
  title = "Étude En Suspens"
  composer = "György Ligeti (1923-2006)"
  tagline = ##f
}

\score
{
  <<
    \new PianoStaff
    {
      <<
        \new Staff = "upper-staff"
        {
          \override Staff.TimeSignature #'stencil = ##f
          \set Score.tempoHideNote = ##t
          \relative c''
          {
            \clef treble
            \time 6/4
            \key df \major
            f4^>_\pp( ef df bf2) 
            <<
              {
                s4 |
                s4*4 f'4^>(\< ef | \stemDown
                <df af'>4 <f gf> <af, bf'>4\p <bf af'>2.)_\pp \stemNeutral
              }
              \\
              {
                <f' gf>4^>( |
                ef4 df bf af gf2) |
                s4*6
              }
            >>

            \key c \major
            <b' c>4^>( a g e2) <g d'>4^>
          }
        }

        \new Staff = "lower-staff"
        {
          \override Staff.TimeSignature #'stencil = ##f
          \relative c''
          {
            \clef treble
            \time 6/4
            \key c \major
            e4.^>_\pp( d4. c4. b4. |
            a2.)
              \shape #'( (0 . -2) (0 . 1) (0 . 0) (0 . -1)) Slur
              <b c>4.(\< <g a'>4.\p |
            d'2.\> <a e'>2.)\pp |

            \key df \major
            <gf af>4.->( gf4. f4. ef4.)
          }
        }
      >>
    }
  >>

  \layout
  {
    \context
    {
      \Score
    }
  }
}

Answer 1

The naturals are printed by a KeyCancellation object, while the flats are a KeySignature. When LilyPond wants to align prefatory matter (like time signatures, clefs, key signatures, etc.), it looks up the break-align-symbol property on each grob (graphical object), and groups grobs according to values of this property. The default is key-cancellation for KeyCancellation and key-signature for KeySignature. By setting the break-align-symbol of KeyCancellation to be the same as that for KeySignature, you can achieve your desired alignment.

\override Staff.KeyCancellation.break-align-symbol = #'key-signature

In the code below, I have also made some simplifications:

• The modern way to spell \include "english.ly" is \language english;
• #(set-global-staff-size 20) is useless, since that is the default;
• \override Something.stencil = ##f can be shortened as \omit Something.

\version "2.22.2"

\language english

\include "articulate.ly"
\pointAndClickOff

\header
{
  title = "Étude En Suspens"
  composer = "György Ligeti (1923-2006)"
  tagline = ##f
}

\new PianoStaff <<
  \new Staff = "upper-staff"
  {
    \omit Staff.TimeSignature
    \set Score.tempoHideNote = ##t
    \relative c'' {
      \clef treble
      \time 6/4
      \once \override Staff.KeyCancellation.break-align-symbol = #'key-signature
      \key df \major
      f4^>_\pp( ef df bf2) 
      <<
        {
          s4 |
          s4*4 f'4^>(\< ef | \stemDown
          <df af'>4 <f gf> <af, bf'>4\p <bf af'>2.)_\pp \stemNeutral
        }
        \\
        {
          <f' gf>4^>( |
          ef4 df bf af gf2) |
          s4*6
        }
      >>

      \key c \major
      <b' c>4^>( a g e2) <g d'>4^>
    }
  }

  \new Staff = "lower-staff" {
    \omit Staff.TimeSignature
    \relative c''
    {
      \clef treble
      \time 6/4
      \key c \major
      e4.^>_\pp( d4. c4. b4. |
      a2.)
        \shape #'( (0 . -2) (0 . 1) (0 . 0) (0 . -1)) Slur
        <b c>4.(\< <g a'>4.\p |
      d'2.\> <a e'>2.)\pp |

      \key df \major
      <gf af>4.->( gf4. f4. ef4.)
    }
  }
>>