Merge master into nested-bookparts

[lilypond.git] / Documentation / user / chords.itely

@c -*- coding: utf-8; mode: texinfo; -*-
@ignore
    Translation of GIT committish: FILL-IN-HEAD-COMMITTISH

    When revising a translation, copy the HEAD committish of the
    version that you are working on.  See TRANSLATION for details.
@end ignore

@c \version "2.11.61"


@node Chord notation
@section Chord notation

@lilypondfile[quote]{chords-headword.ly}

Chords can be entered either as normal notes or in chord mode and displayed
using a variety of traditional European chord naming conventions.  Chord
names and figured bass notation can also be displayed.

@menu
* Chord mode::
* Displaying chords::
* Figured bass::
@end menu


@node Chord mode
@subsection Chord mode

@cindex chord chords

Chord mode is used to enter chords using an indicator of the chord
structure, rather than the chord pitches.

@menu
* Chord mode overview::
* Common chords::
* Extended and altered chords::
@end menu

@node Chord mode overview
@unnumberedsubsubsec Chord mode overview

@cindex chord names
@cindex chord mode

Chords can be entered as simultaneous music, as discussed in
@ref{Chorded notes}.

Chords can also be entered in @qq{chord mode}, which is an input
mode that focuses on the structures of chords in traditional
European music, rather than on specific pitches.  This is
convenient for those who are familiar with using chord names to
describe chords.  More information on different input modes can be
found at @ref{Input modes}.

@lilypond[verbatim,quote,ragged-right,relative=1]
\chordmode { c1 g a g c }
@end lilypond

Chords entered using chord mode are music elements, and can be
transposed just like chords entered using simultaneous music.

Chord mode and note mode can be mixed in sequential music:

@lilypond[verbatim,quote,ragged-right,relative=1]
<c e g>2 <g b d>
\chordmode { c2 f }
<c e g>2 <g' b d>
\chordmode { f2 g }
@end lilypond


@seealso

Music Glossary:
@rglos{chord}.

Notation Reference:
@ref{Chorded notes},
@ref{Input modes}.

Snippets:
@rlsr{Chords}

@knownissues

When chord mode and note mode are mixed in sequential music, and
chord mode comes first, the note mode will create a new @code{Staff}
context.

@lilypond[verbatim,quote,ragged-right,relative=1]
\chordmode { c2 f }
<c e g>2 <g' b d>
@end lilypond

To avoid this behavior, explicitly create the @code{Staff} context:

@lilypond[verbatim,quote,ragged-right,relative=1]
\new Staff {
  \chordmode { c2 f }
  <c e g>2  <g' b d>
}
@end lilypond

@node Common chords
@unnumberedsubsubsec Common chords

@cindex triads
@cindex seventh chords
@cindex root of chord
@cindex modifiers, in chords.
@cindex chord quality

Major triads are entered by including the root and an
optional duration:

@lilypond[verbatim,quote,relative=1,ragged-right]
\chordmode { c2 f4 g }
@end lilypond

Minor, augmented, and diminished triads are entered by placing
@code{:} and a quality modifier string after the duration:

@lilypond[verbatim,quote,ragged-right,relative=1]
\chordmode { c2:m f4:aug g:dim }
@end lilypond

Seventh chords can be created:

@lilypond[quote,ragged-right,fragment,verbatim,relative=1]
\chordmode { c1:7 c:m7 c:maj7 c:dim7 c:aug7 }
@end lilypond

@funindex aug
@funindex dim
@funindex maj
@funindex m

The table belows shows the actions of the quality modifiers on
triads and seventh chords.  A more complete table of modifier usage
is found at @ref{Common chord modifiers}.

@c @table @code
@multitable @columnfractions .2 .4 .3

@item
@b{Modifier}
@tab
@b{Action}
@tab
@b{Example}

@item
None
@tab
The default action; produces a major triad.
@tab
@lilypond[line-width=4\cm, noragged-right]
\chordmode {
  \override Staff.TimeSignature #'stencil = ##f
  c1
}
@end lilypond

@item 
m, m7
@tab
The minor chord.  This modifier lowers the 3rd and (if present) the
7th step.
@tab
@lilypond[line-width=4\cm, noragged-right]
\chordmode {
  \override Staff.TimeSignature #'stencil = ##f
  c1:m c:m7
}
@end lilypond


@item 
dim, dim7
@tab
The diminished chord.  This modifier lowers the 3rd, 5th and (if
present) the 7th step.
@tab
@lilypond[line-width=4\cm, noragged-right]
\chordmode {
  \override Staff.TimeSignature #'stencil = ##f
  c1:dim c:dim7
}
@end lilypond

@item 
aug
@tab
The augmented chord.  This modifier raises the 5th step.
@tab
@lilypond[line-width=4\cm, noragged-right]
\chordmode {
  \override Staff.TimeSignature #'stencil = ##f
  c1:aug
}
@end lilypond

@item 
maj, maj7
@tab
The major 7th chord.  This modifier adds a raised 7th step.  The
@code{7} following @code{maj} is optional.  Do NOT use this modifier
to create a major triad.
@tab
@lilypond[line-width=4\cm, noragged-right]
\chordmode {
  \override Staff.TimeSignature #'stencil = ##f
  c1:maj c:maj7
}
@end lilypond

@end multitable


@seealso

Notation Reference:
@ref{Common chord modifiers}.

Snippets:
@rlsr{Chords}.

@node Extended and altered chords
@unnumberedsubsubsec Extended and altered chords

@cindex extended chords
@cindex altered chords

Chord structures of arbitrary complexity can be created in chord
mode.  The modifier string can be used to extend a chord, add or
remove chord steps, raise or lower chord steps, and add a bass note
or create an inversion.

The first number following the @code{:} is taken to be the extent
of the chord.  The chord is constructed by sequentially adding
thirds to the root until the specified number has been reached.
If the extent is not a third (e.g., 6), thirds are added up to the
highest third below the extent, and then the step of the extent is
added.  The largest possible value for the extent is 13.  Any
larger value is interpreted as 13.

@lilypond[quote,ragged-right,fragment,verbatim,relative=1]
\chordmode {
  c1:2 c:3 c:4 c:5
  c1:6 c:7 c:8 c:9
  c1:10 c:11 c:12 c:13
  c1:14
}
@end lilypond

@noindent
Note that both @code{c:5} and @code{c} produce a C major triad.

Since an unaltered 11 does not sound good when combined with an
unaltered 13, the 11 is removed from a @code{:13} chord (unless it
is added explicitly).

@lilypond[quote,ragged-right,fragment,verbatim,relative=1]
\chordmode {
  c1:13 c:13.11 c:m13
}
@end lilypond

@cindex additions, in chords

Individual steps can be added to a chord.  Additions follow the
extent and are prefixed by a dot (@code{.}).

@lilypond[quote,verbatim,fragment,relative=1]
\chordmode {
  c1:5.6 c:3.7.8 c:3.6.13
}
@end lilypond

Added steps can be as high as desired.

@lilypond[quote,verbatim,fragment,relative=1]
\chordmode {
  c4:5.15 c:5.20 c:5.25 c:5.30
}
@end lilypond

@cindex chord steps, altering

Added chord steps can be altered by suffixing a @code{-} or @code{+}
sign to the number.  To alter a step that is automatically included
as part of the basic chord structure, add it as an altered step.

@lilypond[quote,verbatim,fragment,relative=1]
\chordmode {
  c1:7+ c:5+.3- c:3-.5-.7-
}
@end lilypond

@cindex removals, in chords

@funindex ^

Following any steps to be added, a series of steps to be removed 
is introduced in a modifier string with a prefix of @code{^}.  
If more than one step is to be removed, the steps to be
removed are separated by @code{.} following the
initial @code{^}. 

@lilypond[quote,verbatim,fragment,relative=1]
\chordmode {
  c1^3 c:7^5 c:9^3 c:9^3.5 c:13.11^3.7
}
@end lilypond

@funindex sus

The modifier @code{sus} can be added to the modifier string to
create suspended chords.  This removes the 3rd step from the chord.
Append either @code{2} or @code{4} to add the 2nd or 4th step to the
chord.  @code{sus} is equivalent to @code{^3}; @code{sus4} is
equivalent to @code{.4^3}.

@lilypond[quote,ragged-right,fragment,verbatim]
\chordmode {
  c1:sus c:sus2 c:sus4 c:5.4^3
}
@end lilypond

@funindex /
@cindex chord inversions
@cindex bass note, for chords

Inversions (putting a pitch other than the root on the bottom of the
chord) and added bass notes can be specified by appending
@code{/}@var{pitch} to the chord.

@lilypond[quote,ragged-right,fragment,verbatim, relative=2]
\chordmode {
  c1 c/g c/f
}
@end lilypond

@funindex /+

A bass note that is part of the chord can be added, instead of
moved as part of an inversion, by using @code{/+}@var{pitch}.

@lilypond[quote,ragged-right,fragment,verbatim]
\chordmode {
  c1 c/g c/+g
}
@end lilypond

Chord modifiers that can be used to produce a variety of
standard chords are shown in 
@ref{Common chord modifiers}.

@seealso

Notation Reference:
@ref{Common chord modifiers}.

Snippets:
@rlsr{Chords}

@knownissues

Each step can only be present in a chord once.  The following
simply produces the augmented chord, since @code{5+} is
interpreted last.

@lilypond[quote,ragged-right,verbatim,fragment]
\chordmode { c1:5.5-.5+ }
@end lilypond

Only the second inversion can be created by adding a bass
note.  The first inversion requires changing the root of
the chord.

@lilypond[quote,ragged-right,verbatim,fragment]
\chordmode {
  c'1: c':/g e:6-3-^5 e:m6-^5
}
@end lilypond

@node Displaying chords
@subsection Displaying chords

Chords can be displayed by name, in addition to the standard display
as notes on a staff.

@menu
* Printing chord names::
* Customizing chord names::
@end menu

@node Printing chord names
@unnumberedsubsubsec Printing chord names

@cindex printing chord names
@cindex chord names
@cindex chords

Chord names are printed in the @code{ChordNames} context:

@lilypond[verbatim,quote,relative=1,ragged-right]
\new ChordNames {
  \chordmode {
    c2 f4. g8
  }
}
@end lilypond

Chords can be entered as simultaneous notes or through the use of
chord mode.  The displayed chord name will be the same, regardless
of the mode of entry, unless there are inversions or added bass notes:

@lilypond[verbatim,quote,relative=1]
<<
  \new ChordNames {
    <c e g>2  <f bes c>
    <f c' e g>1
    \chordmode {
      c2 f:sus4 c1:/f
    }
  }
  {
    <c e g>2  <f bes c>
    <f, c' e g>1
    \chordmode {
      c2 f:sus4 c1:/f
    }
  }
>>
@end lilypond

@funindex{\chords}

@code{\chords @{ ... @}} is a shortcut notation for
@code{\new ChordNames @{ \chordmode @{ ... @} @}}.

@lilypond[verbatim,quote,ragged-right, relative=1]
\chords {
  c2 f4.:m g8:maj7
}
@end lilypond

@lilypond[verbatim,quote,ragged-right, relative=1]
\new ChordNames {
  \chordmode {
    c2 f4.:m g8:maj7
  }
}
@end lilypond

@snippets

@lilypondfile[verbatim,lilyquote,ragged-right,texidoc,doctitle]
{showing-chords-at-changes.ly}

@c @lilypondfile[verbatim,lilyquote,ragged-right,texidoc,doctitle]
@c {adding-bar-lines-to-chordnames-context.ly}

@lilypondfile[verbatim,lilyquote,ragged-right,texidoc,doctitle]
{simple-lead-sheet.ly}


@seealso

Music Glossary:
@rglos{chord}.


Notation Reference:
@ref{Writing music in parallel}.


Snippets:
@rlsr{Chords}.

Internals Reference:
@rinternals{ChordNames},
@rinternals{ChordName},
@rinternals{Chord_name_engraver},
@rinternals{Volta_engraver},
@rinternals{Bar_engraver}.

@knownissues

Chords containing inversions or altered bass notes are not named
properly if entered using simultaneous music.


@node Customizing chord names
@unnumberedsubsubsec Customizing chord names

@cindex customizing chord names

There is no unique system for naming chords.  Different musical
traditions use different names for the same set of chords.  There
are also different symbols displayed for a given chord name.   The
names and symbols displayed for chord names are customizable.

@cindex jazz chords
@cindex chords, jazz

The basic chord name layout is a system for Jazz music, proposed
by Klaus Ignatzek (see @ref{Literature list}).  The chord naming
system can be modified as described below.  An alternate jazz
chord system has been developed using these modifications.
The Ignatzek and alternate
Jazz notation are shown on the chart in @ref{Chord
name chart}.

@c TODO --  Change this so we don't have a non-verbatim example.
@c  Make short example in docs, then move longer example to
@c  appendix, where the length of the snippet won't matter.

In addition to the different naming systems, different note names
are used for the root in different languages.  The predefined
variables @code{\germanChords}, @code{\semiGermanChords},
@code{\italianChords} and @code{\frenchChords} set these variables.
The effect is demonstrated here:

@lilypondfile[ragged-right]{chord-names-languages.ly}

If none of the existing settings give the desired output, the chord
name display can be tuned through the following properties.

@table @code

@funindex chordRootNamer

@item chordRootNamer

The chord name is usually printed as a letter for the root with an
optional alteration.  The transformation from pitch to letter is
done by this function.  Special note names (for example, the German
@q{H} for a B-chord) can be produced by storing a new function in
this property.

@funindex majorSevenSymbol

@item majorSevenSymbol

This property contains the markup object used to follow the output
of @code{chordRootNamer} to identify a major 7 chord. Predefined
options are @code{whiteTriangleMarkup} and
@code{blackTriangleMarkup}.

@funindex chordNoteNamer

@item chordNoteNamer

When the chord name contains additional pitches other than the root
(e.g., an added bass note), this function is used to print the
additional pitch.  By default the pitch is printed using
@code{chordRootNamer}. The @code{chordNoteNamer} property can be set
to a specialized function to change this behavior.  For example, the
bass note can be printed in lower case.

@funindex chordNameSeparator

@item chordNameSeparator

Different parts of a chord name are normally separated by a slash.
By setting @code{chordNameSeparator}, you can use any desired markup
for a separator.

@funindex chordNameExceptions

@item chordNameExceptions

This property is a list of pairs.  The first item in each pair
is a set of pitches used to identify the steps present in the chord.
The second item is a markup that will follow the @code{chordRootNamer}
output to create the chord name.

@funindex chordPrefixSpacer
@item chordPrefixSpacer

The @q{m} for minor chords is usually printed immediately to the
right of the root of the chord.  A spacer can be placed between
the root and @q{m} by setting @code{chordPrefixSpacer}.
The spacer is not used when the root is altered.

@end table

@predefined

@funindex major seven symbols
@code{\whiteTriangleMarkup},
@code{\blackTriangleMarkup},
@funindex \germanChords
@code{\germanChords},
@funindex \semiGermanChords
@code{\semiGermanChords},
@funindex \italianChords
@code{\italianChords},
@funindex \frenchChords
@code{\frenchChords}.


@snippets

@cindex exceptions, chord names.
@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{chord-name-exceptions.ly}

@c TODO - tweak snippet to use \blackTriangleMarkup as well
@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{chord-name-major7.ly}

@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{adding-bar-lines-to-chordnames-context.ly}

@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{volta-below-chords.ly}

@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{changing-chord-separator.ly}


@seealso

Notation Reference:
@ref{Chord name chart},
@ref{Common chord modifiers}.

Installed Files:
@file{scm/@/chords@/-ignatzek@/.scm},
@file{scm/@/chord@/-entry@/.scm},
@file{ly/@/chord@/-modifier@/-init@/.ly}.

Snippets:
@rlsr{Chords}.

@c Internals Reference:
@c @r internals{}.

@knownissues

Chord names are determined from both the pitches that are present
in the chord and the information on the chord structure that may
have been entered in @code{\chordmode}.  If the simultaneous pitches
method of entering chords is used, undesired names result from
inversions or bass notes.

@lilypond[quote,ragged-right,verbatim]
myChords = \relative c' {
  \chordmode { c1 c/g c/f }
  <c e g>1 <g c e> <f c' e g>
}
<<
  \new ChordNames { \myChords }
  \new Staff { \myChords }
>>
@end lilypond


@node Figured bass
@subsection Figured bass

@lilypondfile[quote]{figured-bass-headword.ly}

Figured bass notation can be displayed.

@menu
* Introduction to figured bass::
* Entering figured bass::
* Displaying figured bass::
@end menu

@node Introduction to figured bass
@unnumberedsubsubsec Introduction to figured bass

@cindex Basso continuo
@cindex Thorough bass
@cindex Figured bass
@cindex Bass, thorough
@cindex Bass, figured

@c TODO: musicological blurb about FB


LilyPond has support for figured bass, also called thorough bass
or basso continuo:

@lilypond[quote,ragged-right,verbatim,fragment]
<<
  \new Voice { \clef bass dis4 c d ais g fis}
  \new FiguredBass {
    \figuremode {
      < 6 >4 < 7\+ >8 < 6+ [_!] >
      < 6 >4 <6 5 [3+] >
      < _ >4 < 6 5/>4
    }
  }
>>
@end lilypond

The support for figured bass consists of two parts: there is an
input mode, introduced by @code{\figuremode}, that accepts
entry of bass figures, and there is a context named
@code{FiguredBass} that takes care of displaying
@code{BassFigure} objects.  Figured bass can also be displayed
in @code{Staff} contexts.

@code{\figures@{ ... @}} is a shortcut notation for
@code{\new FiguredBass @{ \figuremode @{ ... @} @}}.


Although the support for figured bass may superficially resemble chord
support, it is much simpler.  @code{\figuremode} mode simply
stores the figures and the @code{FiguredBass} context prints them
as entered.  There is no conversion to pitches.

@ignore
Figures are created as markup texts.  Any of the standard markup
properties can be used to modify the display of figures.  For
example, the vertical spacing of the figures may be set with
@code{baseline-skip}.
@end ignore

@seealso

Music Glossary:
@rglos{figured bass}.

Snippets:
@rlsr{Chords}


@node Entering figured bass
@unnumberedsubsubsec Entering figured bass

@code{\figuremode} is used to switch the input mode to figure mode.
More information on different input modes can be
found at @ref{Input modes}.

In figure mode, a group of bass figures is delimited by
@code{<} and @code{>}.  The duration is entered after the @code{>}.

@lilypond[verbatim,quote,ragged-right,fragment]
\new FiguredBass {
  \figuremode {
    <6 4>2
  }
}
@end lilypond


Accidentals (including naturals) can be added to figures:

@lilypond[verbatim,quote,ragged-right,fragment]
\figures {
  <7! 6+ 4-> <5++> <3-->
}
@end lilypond

Augmented and diminished steps can be indicated:

@lilypond[verbatim,quote,ragged-right,fragment]
\figures {
  <6\+ 5/> <7/>
}
@end lilypond

A backward slash through a figure (typically used for raised
sixth steps) can be created:

@lilypond[verbatim,quote,ragged-right,fragment]
\figures {
  <6> <6\\>
}
@end lilypond

Vertical spaces and brackets can be be included in figures:

@lilypond[verbatim,quote,ragged-right,fragment]
\figures {
  <[12 _!] 8 [6  4]>
}
@end lilypond

Any text markup can be inserted as a figure:

@lilypond[verbatim,quote,ragged-right,fragment]
\figures {
  <\markup { \tiny \number 6 \super (1) } 5>
}
@end lilypond

@c NOTE: We need to include notes any time we use extenders to
@c avoid extraneous staff creation due to Staff.use... in
@c \bassFigureExtendersOn

Continuation lines can be used to indicate repeated figures:

@lilypond[verbatim,quote,ragged-right,fragment]
<<
  {
    \clef bass
    e4 d c b,
    e4 d c b,
  }
  \figures {
    \bassFigureExtendersOn
    <6 4>4 <6 3> <7 3> <7 3>
    \bassFigureExtendersOff
    <6 4>4 <6 3> <7 3> <7 3>
  }
>>
@end lilypond

@noindent
In this case, the extender lines replace existing figures,
unless the continuation lines have been explicitly terminated.

@lilypond[verbatim,quote,ragged-right,fragment]
<<
  \figures {
    \bassFigureExtendersOn
    <6 4>4 <6 4> <6\! 4\!> <6 4>
  }
  {
    \clef bass
    d4 d c c
  }
>>
@end lilypond

The table below summarizes the figure modifiers available.

@multitable @columnfractions .1 .5 .4

@item
@b{Modifier}
@tab
@b{Purpose}
@tab
@b{Example}

@item
+, -, !
@tab
Accidentals
@tab
@lilypond[line-width=4\cm]
\figures {
  <7! 6+ 4-> <5++> <3-->
}
@end lilypond

@item
\+, /
@tab
Augmented and diminished steps
@tab
@lilypond[line-width=4\cm]
\figures {
  <6\+ 5/> <7/>
}
@end lilypond

@item
\\
@tab
Raised sixth step
@tab
@lilypond[line-width=4\cm]
\figures {
  <6\\>
}
@end lilypond

@item
\!
@tab
End of continuation line
@tab
@lilypond[line-width=4\cm]
<<
  \figures {
    \bassFigureExtendersOn
    <6 4> <6 4> <6\! 4\!> <6 4>
  }
  {
    \clef bass
    d d c c
  }
>>
@end lilypond

@end multitable



@predefined
@cindex figured bass extender lines
@code{\bassFigureExtendersOn},
@code{\bassFigureExtendersOff}.


@snippets
@lilypondfile[verbatim,lilyquote,texidoc,doctitle]
{changing-the-positions-of-figured-bass-alterations.ly}

@seealso
@c Music Glossary:
@c @rglos{}.

@c Learning Manual:
@c @rlearning{}.

@c Notation Reference:
@c @ref{}.

@c Application Usage:
@c @rprogram{}.

@c Installed Files:
@c @file{}.

Snippets:
@rlsr{Chords}.

Internals Reference:
@rinternals{BassFigure},
@rinternals{BassFigureAlignment},
@rinternals{BassFigureLine},
@rinternals{BassFigureBracket},
@rinternals{BassFigureContinuation},
@rinternals{FiguredBass}.

@c @knownissues

@node Displaying figured bass
@unnumberedsubsubsec Displaying figured bass

Figured bass can be displayed using the @code{FiguredBass} context,
or in most staff contexts.

When displayed in a @code{FiguredBass} context, the vertical location
of the figures is independent of the notes on the staff.

@lilypond[verbatim,ragged-right,fragment,quote]
<<
  \relative c'' {
    c4 c'8 r8 c,4 c'
  }
  \new FiguredBass {
    \figuremode {
      <4>4 <10 6>8 s8
      <6 4>4 <6 4>
    }
  }
>>
@end lilypond

@noindent
In the example above, the @code{FiguredBass} context must be
explicitly instantiated to avoid creating a second (empty) staff.


Figured bass can also be added to @code{Staff} contexts
directly.  In this case, the vertical position of the
figures is adjusted automatically.

@lilypond[verbatim,ragged-right,fragment,quote]
<<
  \new Staff = myStaff
  \figuremode {
    <4>4 <10 6>8 s8
    <6 4>4 <6 4>
  }
  %% Put notes on same Staff as figures
  \context Staff = myStaff
  {
    \clef bass
    c4 c'8 r8 c4 c'
  }
>>
@end lilypond


When added in a @code{Staff} context, figured bass can be displayed above
or below the staff.

@lilypond[verbatim,ragged-right,fragment,quote]
<<
  \new Staff = myStaff
  \figuremode {
    <4>4 <10 6>8 s8
    \bassFigureStaffAlignmentDown
    <6 4>4 <6 4>
  }
  %% Put notes on same Staff as figures
  \context Staff = myStaff
  {
    \clef bass
    c4 c'8 r8 c4 c'
  }
>>
@end lilypond


@predefined
@cindex figured bass alignment
@code{\bassFigureStaffAlignmentDown},
@code{\bassFigureStaffAlignmentUp},
@code{\bassFigureStaffAlignmentNeutral}.

@c @snippets

@seealso
@c Music Glossary:
@c @rglos{}.

@c Learning Manual:
@c @rlearning{}.

@c Notation Reference:
@c @ref{}.

@c Application Usage:
@c @rprogram{}.

@c Installed Files:
@c @file{}.

Snippets:
@rlsr{Chords}.

Internals Reference:
@rinternals{BassFigure},
@rinternals{BassFigureAlignment},
@rinternals{BassFigureLine},
@rinternals{BassFigureBracket},
@rinternals{BassFigureContinuation},
@rinternals{FiguredBass}.

@knownissues

To ensure that continuation lines work properly, it is
safest to use the same rhythm in the figure line as in
the bass line.

@lilypond[verbatim,ragged-right,fragment,quote]
<<
  {
    \clef bass
    \repeat unfold 4 { f16. g32 } f8. es16 d8 es
  }
  \figures {
    \bassFigureExtendersOn
    % The extenders are correct here, with the same rhythm as the bass
    \repeat unfold 4 { <6 4->16. <6 4->32 }
    <5>8. r16 <6>8 <6\! 5->
  }
>>
<<
  {
    \clef bass
    \repeat unfold 4 { f16. g32 } f8. es16 d8 es
  }
  \figures {
    \bassFigureExtendersOn
    % The extenders are incorrect here, even though the timing is the same
    <6 4->4 <6 4->4
    <5>8. r16 <6>8 <6\! 5->
  }
>>
@end lilypond

When using extender lines, adjacent figures with the same number in
a different figure location can cause the figure positions to invert.

@lilypond[verbatim,ragged-right,fragment,quote,relative=1]
<<
 { fis4 g g, e' }
  \figures {
    \bassFigureExtendersOn
    <6 5>4 <5\! 4> < 5 _!> <6>
  }
>>
@end lilypond

To avoid this problem, simply turn on extenders after the figure that
begins the extender line and turn them off at the end of the extender line.

@lilypond[verbatim,ragged-right,fragment,quote,relative=1]
<<
 { fis4 g g, e' }
  \figures {
    <6 5>4 <5 4>
    \bassFigureExtendersOn
    < 5 _!>4 <6>
    \bassFigureExtendersOff
  }
>>
@end lilypond