Re: Is there a better way to have a boxed part letter at the beginning of an arbitrary line?

[Michael Werner]

Hi Kevin.

It sounds like you might be able to do want you want with rehearsal marks. They're built into Lilypond and do pretty much what you seem to be describing. Details at http://lilypond.org/doc/v2.24/Documentation/notation/bars#index-rehearsal-mark

A modification of your MWE using them:

\version "2.24.2"
\language "english"

melody = \relative c' {
  \set Score.rehearsalMarkFormatter = #format-mark-box-letters
  \time 6/8
  \key d \major
  \clef treble

  \mark \default
  d8[ e8 e8] g8[ a8 b8] |
  \break
  \mark \default
  d8[ e8 e8] g8[ a8 b8] |
  \fine
}

\score {
  \new Voice = "mel" { \melody }

  \layout {
    \autoBeamOff
    indent = 0\mm
  }
}

The \set Score.rehearsalMarkFormatter = #format-mark-box-letters part reformats the marks to display as letters inside a box - the default is just a letter. By doing \default after the \mark Lilypond simply uses the letters sequentially. If needed, the position of the marks can be adjusted. Again, details on that are in the page linked above.

-- 

Michael