Egger and Miller found that when one CS precedes another CS and they both precede an US:

Question

Quizplus · Accepted Answer

Egger and Miller's research demonstrated that when two conditioned stimuli (CS) are presented in sequence before an unconditioned stimulus (US), the first CS becomes associated with the US, while the second CS does not. This is due to the phenomenon of blocking, where the first CS blocks the conditioning of the second CS.

Egger and Miller Found That When One CS Precedes Another