When a beam falls in the middle of the staff, the beams point normally
down. However, this behaviour can be altered with the
neutral-direction property.
{
b8[ b]
\override Beam #'neutral-direction = #-1
b[ b]
\override Beam #'neutral-direction = #1
b[ b]
}
This page is for LilyPond-2.4.5 (stable-branch).