The conceptual heart of the semiotic square is the fact that a given term (for instance day ) is not only opposed to its contrary (in this case: night ), but also to its contradictory term (which is non-day ). The semiotic difference between contrary and contradictory is the following one: whereas, on the so-called conjunction/disjunction axis, a term and its contrary term may in a certain sense be mixed up or being superposed (twilight for instance is something which is neither night nor day), a term and its contradictory term, which are situated on the so-called contradiction axis, are necessarily mutually exclusive (it is not possible for a term to belong simultaneously to day and non-day).