#### 4.2.4 Layout tunings within contexts

Each context is responsible for creating certain types of graphical objects. The settings used for printing these objects are also stored by context. By changing these settings, the appearance of objects can be altered.

The syntax for this is

           \override context.name #'property = #value

Here name is the name of a graphical object, like `Stem` or `NoteHead`, and property is an internal variable of the formatting system (`grob property' or `layout property'). The latter is a symbol, so it must be quoted. The subsection Constructing a tweak explains what to fill in for name, property, and value. Here we only discuss functionality of this command.

The command

      \override Staff.Stem #'thickness = #4.0

makes stems thicker (the default is 1.3, with staff line thickness as a unit). Since the command specifies `Staff` as context, it only applies to the current staff. Other staves will keep their normal appearance. Here we see the command in action:

      c4
      \override Staff.Stem #'thickness = #4.0
      c4
      c4
      c4

The `\override` command is executed during the interpreting phase, and changes the definition of the `Stem` within `Staff` . After the command all stems are thickened.

Analogous to `\set`, the context argument may be left out, causing it to default to `Voice` , and adding `\once` applies the change during one timestep only

      c4
      \once \override Stem #'thickness = #4.0
      c4
      c4

The `\override` must be done before the object is started. Therefore, when altering Spanner objects, like slurs or beams, the `\override` command must be executed at the moment when the object is created. In this example,

      \override Slur #'thickness = #3.0
      c8[( c
      \override Beam #'thickness = #0.6
      c8 c])

the slur is fatter and the beam is not. This is because the command for `Beam` comes after the Beam is started. Therefore it has no effect.

Analogous to `\unset`, the `\revert` command for a context undoes a `\override` command; like with `\unset`, it only affects settings that were made in the same context. In other words, the `\revert` in the next example does not do anything.

      \override Voice.Stem #'thickness = #4.0
      \revert Staff.Stem #'thickness

##### See also

Internals: OverrideProperty, RevertProperty, PropertySet, All-backend-properties, and All-layout-objects.

##### Bugs

The back-end is not very strict in type-checking object properties. Cyclic references in Scheme values for properties can cause hangs or crashes, or both.

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