The basic idea of the Rescorla-Wagner model of respondent conditioning is that a conditioned stimulus acquires a limited amount of associative strength on any one trial. The term associative strength describes the relation between the CS and the magnitude of the conditioned response. In general, associative strength increases over conditioning trials and reaches some maximum level. A given CS can acquire only so much control over a conditioned response. This is the maximum associative strength for the CS. Thus, a tone (CS) that is paired with I gramme of food will have maximum associative strength when the amount of conditioned salivation (CR) is about the same as the unconditioned salivation (UR) elicited by the gramme of food (US). This magnitude sets the upper limit for the conditioned response. The CS cannot elicit a greater response than the one produced by the unconditioned stimulus.