Typographical Number Theory (TNT) is a formal axiomatic system describing the natural numbers that appears in Douglas Hofstadter's book Gödel, Escher, Bach. It is an implementation of Peano arithmetic that Hofstadter uses to help explain Gödel's incompleteness theorems. Like any system implementing the Peano axioms, TNT is capable of referring to itself (it is self-referential). Source: Wikipedia (en)