Creating trill staff in lilypond

Question

I'm learning BWV775 (Bach Invention IV - D Minor) and the score I'm working from has a small extra staff for a trill in measure 17

The treble & bass staves are easy enough in Lilypond, and for the trill mark, I've just used /trill in the treble voice

f-1[ g] g8.[ \trill f16]

which gives me

What's the best approach to add a staff at this measure? Create some sort of new staff with \context Staff in the voice at that point?

My piano scores have been pretty simple to date using

\score {
    \context GrandStaff <<
     \context Staff = "one" <<
       \voiceone
     >>
     \context Staff = "two" <<
       \voicetwo
     >>
   >>

Answer 1

The documentation calls this an "Ossia staff" (see here).

The main idea is to make a new staff \with an alignAboveContext that links it to the upper staff.

• In the example in the documentation they use firstClef = ##f to hide the clef symbol in this new staff, but this isn't done in the image you are trying to replicate.

• To hide the time signature use: \remove "Time_signature_engraver".

• To scale the new staff use: \magnifyStaff #2/3
  This will also make the bar lines thinner, so you’ll want to revert them to their usual thickness by using:
  \revert BarLine.hair-thickness
\revert BarLine.thick-thickness
  which prevents a misalignments issue with the normal bar lines.

• You might also want to reduce the distance to the ossia staff with:
  \override VerticalAxisGroup.staff-staff-spacing.minimum-distance = 2
\override VerticalAxisGroup.staff-staff-spacing.basic-distance = 8

• To get the lower mordent symbol use \prall rather than trill.

You should get something like this:

%% GRAND STAFF
\new GrandStaff {
    <<
        %% UPPER STAFF
        \new Staff = "upper" {
            \key f \major
            \time 3/8
            f'16[ g']
            <<
                {
                    g'8.[\prall f'16] |
                }
                %% OSSIA
                \new Staff \with {
                    alignAboveContext = "upper"
                    %firstClef = ##f
                    \remove "Time_signature_engraver"
                    \magnifyStaff #2/3
                    \revert BarLine.hair-thickness
                    \revert BarLine.thick-thickness
                    \override VerticalAxisGroup.staff-staff-spacing.minimum-distance = 5
                    \override VerticalAxisGroup.staff-staff-spacing.basic-distance = 8
                } {
                    \key f \major
                    \tuplet 3/2 {a'32 g' a'} g'8 f'16 |
                }
            >>
            f'8
        }
        %% LOWER STAFF
        \new Staff = "lower" {
            \clef "bass"
            \key f \major
            \time 3/8
            d16 bes, c8 c, |
            f,16 g,
        }
    >>
}