A conditioned stimulus, after having acquired the ability to elicit CR may be used as a UCS for further conditioning. This phenomenon is called higher order conditioning.
In one experiment, Pavlov fist conditioned a dog to salivate to the beat of a metronome, using meat powder as the UCS. This was first-order conditioning. The metronome beat acquired reinforcing properties and became capable of evoking CR. In the next phase, Pavlov presented the animal with a black square followed by the metronome beat but without food. Eventually, the sight of the black square produced salivation. Thus, a second order conditioning was established. In this manner, Pavlov succeeded in establishing third order conditioning but only when unpleasant stimuli like electric shock was used as the UCS. Food is a primary reinforcer. The metronome beat, by being associated with food acquired some reinforcing properties. The metronome beat is regarded as the secondary reinforcer.
In most cases, high order conditioning is fairly weak. The CR is second-order conditioning was about half the strength other first-order. Third order conditioning was difficult to establish, while forth-order conditioning was impossible altogether.