lilypond-usage: Typesetting existing music
5.2 Typesetting existing music
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If you are entering music from an existing score (i.e., typesetting a
piece of existing sheet music),
• Enter the manuscript (the physical copy of the music) into LilyPond
one system at a time (but still only one bar per line of text), and
check each system when you finish it. You may use the
‘showLastLength’ or ‘showFirstLength’ properties to speed up
processing – see 
(lilypond-notation)Skipping corrected
music.
• Define ‘mBreak = { \break }’ and insert ‘\mBreak’ in the input file
whenever the manuscript has a line break. This makes it much
easier to compare the LilyPond music to the original music. When
you are finished proofreading your score, you may define ‘mBreak =
{ }’ to remove all those line breaks. This will allow LilyPond to
place line breaks wherever it feels are best.
• When entering a part for a transposing instrument into a variable,
it is recommended that the notes are wrapped in
\transpose c natural-pitch {...}
(where ‘natural-pitch’ is the open pitch of the instrument) so that
the music in the variable is effectively in C. You can transpose it
back again when the variable is used, if required, but you might
not want to (e.g., when printing a score in concert pitch,
converting a trombone part from treble to bass clef, etc.)
Mistakes in transpositions are less likely if all the music in
variables is at a consistent pitch.
Also, only ever transpose to/from C. That means that the only other
keys you will use are the natural pitches of the instruments - bes
for a B-flat trumpet, aes for an A-flat clarinet, etc.
Info Catalog
lilypond-usage: General suggestions
lilypond-usage: Suggestions for writing files
lilypond-usage: Large projects