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o The Xt Intrinsics implement an object oriented interface to C code
  to allow useful graphical components to be created.  Included with
  this are classes that provide the base functionality: Object,
  RectObj, Core, Composite, Constraint, Shell, OverrideShell, WMShell,
  etc.  The terms "Xt" and "Intrinsics" are used interchangeably,
  however, they are used very precisely to mean a specific library of the X
  window system.  In particular, it does not include the Athena,
  Motif, OLIT or any other widget set.  Without further widgets the
  Intrinsics are not especially useful.

o A widget refers to a user interface abstraction created via Xt.  The
  precise use, is any object that is a subclass of the Core class.  It
  is used loosely to refer to anything that is a subclass of the
  Object class although these are more accurately called windowless
  widgets or gadgets.

o Xlib is the C interface to the X11 protocol.  It is one layer below
  the Xt Intrinsics.  Typically a widget uses relatively few Xlib
  functions because Xt provides most such services although an
  understanding of Xlib helps with problems.