Default direction of stems on the center line of the staff

The default direction of stems on the center line of the staff is set by the Stem property neutral-direction.

\relative c'' {
  a4 b c b
  \override Stem.neutral-direction = #up
  a4 b c b
  \override Stem.neutral-direction = #down
  a4 b c b
}

[image of music]

LilyPond snippets v2.25.20 (development-branch).