eigenclass

String.class_eval             { def instance_method;   "instance method";   end }
String.class.class_eval       { def class_method;      "class method";      end }
String.class.class.class_eval { def eigenclass_method; "eigenclass method"; end }

"a".instance_method # => "instance method"
String.class_method # => "class method"
class X < String
  eigenclass_method
end                 # => "eigenclass method"