A slur indicates that notes are to be played bound or legato.
They are entered using parentheses
f( g a) a8 b( a4 g2 f4)
<c e>2( <b d>2)
The direction of a slur can be set with the generic commands
\override Slur #'direction = #UP \slurUp % shortcut for the previous line
However, there is a convenient shorthand for forcing slur
directions. By adding _ or ^ before the opening
parentheses, the direction is also set. For example,
c4_( c) c^( c)
Some composers write two slurs when they want legato chords. This can
be achieved in LilyPond by setting doubleSlurs,
\set doubleSlurs = ##t
<c e>4 ( <d f> <c e> <d f> )
\slurUp,
\slurDown,
\slurNeutral,
\slurDotted,
\slurSolid.
Program reference: Slur, and SlurEvent.
This page is for LilyPond-2.4.5 (stable-branch).