+OBjects}, or @strong{grobs} for short. They are represented by
+instances of the @code{Grob} class.
+
+@seealso
+Learning Manual:
+@rlearning{Objects and interfaces},
+@rlearning{Naming conventions of objects and properties},
+@rlearning{Properties of layout objects}.
+
+Internals Reference:
+@rinternals{grob-interface},
+@rinternals{All layout objects}.
+
+
+@node immutable
+@unnumberedsubsec immutable
+
+@cindex immutable objects
+@cindex immutable properties
+@cindex shared properties
+
+An @strong{immutable} object is one whose state cannot be modified
+after creation, in contrast to a mutable object, which can be
+modified after creation.
+
+In LilyPond, immutable or shared properties define the default
+style and behavior of grobs. They are shared between many objects.
+In apparent contradiction to the name, they can be changed using
+@code{\override} and @code{\revert}.
+
+@seealso
+Notation Reference:
+@ref{mutable}.
+
+
+@node interface
+@unnumberedsubsec interface
+
+@cindex interface
+@cindex grob-interface
+@cindex graphical object interfaces
+
+Actions and properties which are common to a number of grobs are
+grouped together in an object called a @code{grob-interface}, or
+just @q{interface} for short.
+
+@seealso
+Learning Manual:
+@rlearning{Objects and interfaces},
+@rlearning{Naming conventions of objects and properties},
+@rlearning{Properties found in interfaces}.
+
+Notation Reference:
+@ref{Layout interfaces}.
+
+Internals Reference:
+@rinternals{Graphical Object Interfaces}.
+