Variables make rule-based systems more abstract and expressive, as witnessed
 by term rewriting systems and two-level grammars.  Variables can be used to
 define advanced ways of graph transformation as well.  In this paper we
 start from the gluing approach to graph
 transformation, and consider
 extensions of rules with attribute variables, clone variables, and graph
 variables, respectively.  In each case, the variables in a rule are
 instantiated in order to obtain a set of rule instances that in turn defines
 the transformation relation.  By combining different kinds of variables, we
 define very expressive rules, and reduce them to plain rules by
 instantiation.  Since gluing graph transformation has a
 well developed theory, this opens the door to lift results of that theory from
 instances to rules with variables.