Creating custom key signatures

LilyPond supports custom key signatures. In this example, print for D minor and D major with an extended range of shown flats.

\version "2.24"

\new Staff \with {
  \override StaffSymbol.line-count = #8
  \override KeySignature.flat-positions = #'((-7 . 6))
  \override KeyCancellation.flat-positions = #'((-7 . 6))
  \override KeySignature.sharp-positions = #'((-6 . 7))
  \override KeyCancellation.sharp-positions = #'((-6 . 7))

  \override Clef.stencil =
    #(lambda (grob)
        (grob-interpret-markup grob
           #{ \markup\combine
                \musicglyph "clefs.C"
                \translate #'(-3 . -2)
                  \musicglyph "clefs.F"
           #}))
    clefPosition = #3
    middleCPosition = #3
    middleCClefPosition = #3
}

{
  \key d\minor f bes, f bes, |
  \key d\major fis b, fis b, |
}