“Squares have four sides.” is necessary.
“Stop signs are hexagonal.” is contingent.
“Pentagons are round.” is contradictory.
This distinction was traditionally associated (before Kant and Kripke) with the distinctions between a priori and a posteriori knowledge and the distinction between analytic and synthetic judgment. Necessity may also be defined de dicto in terms of the formal logical property of tautology.1
1. Philosophy Pages.