Two age groups of infants were tested for their ability to learn an AAB or ABA repetition generalization in sequences of musical chords. The 4-month-olds, but not the 7.5-month-olds, successfully learned the generalization. Another group of 7.5-month-old infants successfully learned a generalization across melodies that all ended on a particular scale degree, even though the key of the melodies was varied. A survey of a musical corpus of children's songs reveals that AAB and ABA patterns do not occur more frequently than chance, while phrases frequently end on particular scale degrees. Together, these findings suggest that infants learn to constrain the set of generalizations they consider in order to favor those that rely upon features of the input that have proved reliable in their previous experience, specifically experience with a particular input domain. This raises the possibility that experience may play a significant role in parsing infants' environments into domains.