\version "2.18.2"

music = \new Staff {
  gis'1
  <<
    \tag #'(a c) c'
    \tag #'(b d) bis
  >>
}

seqChanges = \new ChordNames {

  \chordmode { gis1:maj7 }
  \tag #'a \relative c' <c e g>
  \tag #'b \relative c' <bis disis fisis>
  \tag #'c \chordmode { c1 }
  \tag #'d \chordmode { bis1 }

}

simChanges = \new ChordNames {

  \chordmode { gis1:maj7 }
  <<
    \tag #'a \relative c' <c e g>
    \tag #'b \relative c' <bis disis fisis>
    \tag #'c \chordmode { c1 }
    \tag #'d \chordmode { bis1 }
  >>

}

\markup "Sequential tags make the music expressions have unequal length"
\score {
  <<
    \seqChanges
    \music
  >>
}

\markup "Simultaneous tags make the music expressions the same length"
\score {
  <<
    \simChanges
    \music
  >>
}

\markup "Tag a"
\score {
  <<
    \keepWithTag #'a \simChanges
    \keepWithTag #'a \music
  >>
}

\markup "Tag b"
\score {
  <<
    \keepWithTag #'b \simChanges
    \keepWithTag #'b \music
  >>
}

\markup "Tag c"
\score {
  <<
    \keepWithTag #'c \simChanges
    \keepWithTag #'c \music
  >>
}

\markup "Tag d"
\score {
  <<
    \keepWithTag #'d \simChanges
    \keepWithTag #'d \music
  >>
}