-For some reason, music expressions like @code{<< @{ g8 e8 @} a4 >>}
-that should automatically turn into chords, appear split in two
-staves. To avoid this, use explicit chords instead as in
-@code{<g a>8 <e a>8}.
-@end ignore
+@node Simultaneous expressions
+@unnumberedsubsubsec Simultaneous expressions
+
+One or more music expressions enclosed in double angle brackets are
+taken to be simultaneous. If the first expression begins with a
+single note or if the whole simultaneous expression appears
+explicitly within a single voice, the whole expression is placed on
+a single staff; otherwise the elements of the simultaneous
+expression are placed on separate staves.
+
+The following examples show simultaneous expressions on one staff:
+
+@lilypond[quote,verbatim,relative=2]
+\new Voice { % explicit single voice
+ << {a4 b g2} {d4 g c,2} >>
+}
+@end lilypond
+
+@lilypond[quote,verbatim,relative=2]
+% single first note
+a << {a4 b g} {d4 g c,} >>
+@end lilypond
+
+This can be useful if the simultaneous sections have identical
+rhythms, but attempts to attach notes with different durations
+to the same stem will cause errors.
+
+The following example shows how simultaneous expressions can
+generate multiple staves implicitly:
+
+@lilypond[quote,verbatim,relative=2]
+% no single first note
+<< {a4 b g2} {d4 g2 c,4} >>
+@end lilypond
+
+Here different rhythms cause no problems.