Implied terms: courts imply only terms necessary to give effect to parties' intention, not merely to improve contracts. Implied terms are admitted only where the parties must necessarily have intended them, applying a test of necessity rather than mere reasonableness; courts will not imply terms inconsistent with the express contract and are reluctant to do so in carefully drafted written agreements. Implied terms arise by custom, by law, or by the courts, with the distinctions often overlapping and justified by reference to party intention and convention.