In a typical transposition task, an animal is presented with a single pair of stimuli (for example, S3+S4-, where plus and minus denote reward and nonreward and digits denote stimulus location on a sensory dimension such as size). Subsequently, an animal is presented with a testing pair that contains a previously reinforced or nonreinforced stimulus and a novel stimulus (for example, S2-S3 and S4-S5). Does the choice of a novel S2 instead of previously reinforced S3 in a testing pair S2-S3 indicate that the animal has learned a relation (i.e., "select smaller")? This review of empirical evidence and theoretical accounts shows that an organism's behavior in a transposition task is undoubtedly influenced by prior reinforcement history of the training stimuli (Spence, 1937). However, it is also affected by two other factors that are relational in nature--a similarity of two testing stimuli to each other and an overall similarity of the testing pair as a whole to the training pair as a whole. The influence of the two latter factors is especially evident in studies that use multiple pairs of training stimuli and a wide range of testing pairs comprising nonadjacent stimuli (Lazareva, Miner, Young, & Wasserman, 2008; Lazareva, Wasserman, & Young, 2005). In sum, the evidence suggests that both prior reinforcement history and relational information affect an animal's behavior in a typical transposition task. (Contains 2 footnotes and 12 figures.)