### 4.1.4 Tweaking methods

\override command

We have already met the commands \set and \with, used to change the properties of contexts and to remove and add engravers, in Modifying context properties, and Adding and removing engravers. We must now introduce some more important commands.

The command to change the properties of layout objects is \override. Because this command has to modify internal properties deep within LilyPond its syntax is not as simple as the commands you have used so far. It needs to know precisely which property of which object in which context has to be modified, and what its new value is to be. Let’s see how this is done.

The general syntax of this command is:

\override Context.LayoutObject #'layout-property =
#value


This will set the property with the name layout-property of the layout object with the name LayoutObject, which is a member of the Context context, to the value value.

The Context can be omitted (and usually is) when the required context is unambiguously implied and is one of lowest level contexts, i.e., Voice, ChordNames or Lyrics, and we shall omit it in many of the following examples. We shall see later when it must be specified.

Later sections deal comprehensively with properties and their values, see Types of properties. But in this section we shall use just a few simple properties and values which are easily understood in order to illustrate the format and use of these commands.

For now, don’t worry about the #', which must precede the layout property, and the #, which must precede the value. These must always be present in exactly this form. This is the most common command used in tweaking, and most of the rest of this chapter will be directed to presenting examples of how it is used. Here is a simple example to change the color of the note head:

c4 d
e4 f |
g4 a b c |


\revert command

Once overridden, the property retains its new value until it is overridden again or a \revert command is encountered. The \revert command has the following syntax and causes the value of the property to revert to its original default value; note, not its previous value if several \override commands have been issued.

\revert Context.LayoutObject #'layout-property


Again, just like Context in the \override command, Context is often not needed. It will be omitted in many of the following examples. Here we revert the color of the note head to the default value for the final two notes:

c4 d
e4 f |
g4 a
b4 c |


\once prefix

Both the \override and the \set commands may be prefixed by \once. This causes the following \override or \set command to be effective only during the current musical moment before the property reverts back to its previous value (this can be different from the default if another \override is still in effect). Using the same example, we can change the color of a single note like this:

c4 d
e4 f |
\once \override NoteHead #'color = #green
g4 a
b c |


\overrideProperty command

There is another form of the override command, \overrideProperty, which is occasionally required. We mention it here for completeness, but for details see Difficult tweaks.

\tweak command

The final tweaking command which is available is \tweak. This should be used when several objects occur at the same musical moment, but you only want to change the properties of selected ones, such as a single note within a chord. Using \override would affect all the notes within a chord, whereas \tweak affects just the following item in the input stream.

Here’s an example. Suppose we wish to change the size of the middle note head (the E) in a C major chord. Let’s first see what \once \override would do:

<c e g>4
\once \override NoteHead #'font-size = #-3
<c e g>4
<c e g>4


We see the override affects all the note heads in the chord. This is because all the notes of a chord occur at the same musical moment, and the action of \once is to apply the override to all layout objects of the type specified which occur at the same musical moment as the \override command itself.

The \tweak command operates in a different way. It acts on the immediately following item in the input stream. In its simplest form, it is effective only on objects which are created directly from the following item, essentially note heads and articulations.

So to return to our example, the size of the middle note of a chord would be changed in this way:

<c e g>4
<c \tweak #'font-size #-3 e g>4


Note that the syntax of \tweak is different from that of the \override command. The context should not be specified; in fact, it would generate an error to do so. Both context and layout object are implied by the following item in the input stream. Note also that an equals sign should not be present. So the simple form of the \tweak command is

\tweak #'layout-property #value


A \tweak command can also be used to modify just one in a series of articulations, as shown here:

a4^"Black"
-\tweak #'color #red ^"Red"
-\tweak #'color #green _"Green"


Note that the \tweak command must be preceded by an articulation mark since the tweaked expression needs to be applied as an articulation itself. In case of multiple direction overrides (^ or _), the leftmost override wins since it is applied last.

Objects such as stems and accidentals are created later, and not directly from the following event. It is still possible to use \tweak on such indirectly created objects by explicitly naming the layout object, provided that LilyPond can trace its origin back to the original event:

<\tweak Accidental #'color #red   cis4
\tweak Accidental #'color #green es
g>


This long form of the \tweak command can be described as

\tweak layout-object #'layout-property value


The \tweak command must also be used to change the appearance of one of a set of nested tuplets which begin at the same musical moment. In the following example, the long tuplet bracket and the first of the three short brackets begin at the same musical moment, so any \override command would apply to both of them. In the example, \tweak is used to distinguish between them. The first \tweak command specifies that the long tuplet bracket is to be placed above the notes and the second one specifies that the tuplet number is to be printed in red on the first short tuplet bracket.

\tweak #'direction #up
\times 4/3 {
\tweak #'color #red
\times 2/3 { c8[ c c] }
\times 2/3 { c8[ c c] }
\times 2/3 { c8[ c c] }
}


If nested tuplets do not begin at the same moment, their appearance may be modified in the usual way with \override commands:

\times 2/3 { c8[ c c] }
\once \override TupletNumber
#'text = #tuplet-number::calc-fraction-text
\times 2/3 {
c8[ c]
c8[ c]
\once \override TupletNumber #'transparent = ##t
\times 2/3 { c8[ c c] }
\times 2/3 { c8[ c c] }
}