+@funindex measureLength
+@funindex beatLength
+@funindex beatGrouping
+@funindex \time
+@funindex time
+@funindex \set
+@funindex set
+
+The placement of automatic beams is determined by the rules
+described in @ref{Automatic beams}. There are two mutually
+exclusive ways in which these rules may be modified. The
+first, modifying the grouping of beats, applies to uncommon time
+signatures, i.e. those for which there are no predefined rules
+defining the beam end points. The second method, modifying the
+specification of the beam end points, can be used for any time
+signature. This second method @strong{must} be used for those time
+signatures and beam durations combinations for which beam ending
+rules are pre-defined, unless these have all been reverted. There
+are predefined rules for time signatures of 3/2, 3/4, 4/4, 2/4,
+4/8, 4/16, 6/8, 9/8 and 12/8.
+
+@i{@strong{Modifying the grouping of beats}}
+
+If there are no beam-ending rules defined for the beam duration of a
+particular beam in the time signature in use, its beaming is
+controlled by three context properties:
+@code{measureLength}, @code{beatLength} and @code{beatGrouping}.
+These properties may be set in the @code{Score}, @code{Staff} or
+@code{Voice} contexts to delimit their scope. The default values
+are set while processing @code{\time} commands, so the @code{\set}
+commands must be placed after all @code{\time} commands.
+
+These determine the beaming as follows:
+
+Beams may begin anywhere (unless a beam is already active). Beams
+end at a time determined by the values of @code{beatGrouping} and
+@code{beatLength}, as follows:
+
+@itemize
+
+@item If @code{beatGrouping} and @code{beatLength} are consistent
+with @code{measureLength}, @code{beatGrouping} is used to determine
+the end points of beams.
+
+@item If @code{beatGrouping} and @code{beatLength} are inconsistent
+with @code{measureLength}, @code{beatLength} is used to determine
+the end points of beams.
+
+@end itemize
+
+@warning{These three properties become effective for a particular
+beam @strong{only} if there are no beam-ending rules predefined for
+that beam's duration in the time signature in use, or if these
+beam-ending rules have all been reverted.}
+
+By default the @code{measureLength} and @code{beatLength} are
+derived from the time signature set by the @code{\time} command.
+The @code{measureLength} is set to be exactly the same length as
+the measure length given by the time signature, and the
+@code{beatLength} is set to be the same as one over the denominator
+of the time signature.
+
+The default value of @code{beatGrouping} is taken from a table in
+@file{scm/@/music@/-functions@/.scm}. To find this, see
+@rlearning{Other sources of information}. It defines the beat
+grouping for 5/8, 6/8, 8/8, 9/8 and 12/8 time signatures.
+
+Both @code{measureLength} and @code{beatLength} are @i{moments},
+units of musical duration. A quantity of type @i{moment} is
+created by the scheme function @code{ly:make-moment}. For more
+information about this function, see @ref{Time administration}.