true, false and nil
TrueClass
The global value true is the only instance of class TrueClass and represents a logically true value in boolean expressions. The class provides operators allowing true to be used in logical expressions.
FalseClass
The global value false is the only instance of class FalseClass and represents a logically false value in boolean expressions. The class provides operators allowing false to participate correctly in logical expressions.
NilClass
The class of the singleton object nil.