Schematic diagram of the typical classical conditioning procedure.
Pairing a conditioned stimulus (CS e.g. a bell) with an unconditioned stimulus
 (UCS or US, e.g. food) will typically, after a number of pairings, mean that
 a conditioned response (CR, in this case salivation) is evoked by the CS 
 (redrawn after figure 2.1 in Davey, 1989).

Schematic diagram of the typical classical conditioning procedure. Pairing a conditioned stimulus (CS e.g. a bell) with an unconditioned stimulus (UCS or US, e.g. food) will typically, after a number of pairings, mean that a conditioned response (CR, in this case salivation) is evoked by the CS (redrawn after figure 2.1 in Davey, 1989).