We test whether comparison can promote learning of new relational abstractions. In Experiment 1, preschoolers heard labels for novel spatial patterns and were asked to extend the label to one of two alternatives: one sharing an object with the standard or one having the same relational pattern as the standard. Children strongly preferred the object match when given one standard but were significantly more likely to choose the relational match when they compared two standards. Experiment 2 provided evidence that comparison processing--as opposed to simply seeing two exemplars--is necessary for this relational effect. Preschoolers who were shown the two standards sequentially without a prompt to compare them preferred object matches, as did those who viewed only one standard. In contrast, those who saw the exemplars together, with a prompt to compare them, showed the same elevated relational responding as found in Experiment 1. We suggest that structural alignment processes are crucial to developing new relational abstractions. (Contains 4 figures and 2 footnotes.)