One of the most important findings in recent years regarding response selection is that stimulus-response compatibility (SRC) effects occur for situations in which stimulus and response sets vary along orthogonal dimensions. For two-choice tasks, two types of orthogonal SRC effects are found: an overall advantage for the up-right/down-left mapping, and mapping preferences that vary as a function of position of the response apparatus and responding hand. We review evidence regarding the nature of both types of orthogonal SRC effects. Only asymmetric coding accounts have been proposed for the up-right/down-left advantage, and the evidence indicates that this asymmetry is a property of both verbal and spatial codes. Motoric and coding accounts, as well as a hybrid account based on end-state comfort, have been proposed for the second type of orthogonal SRC effect. In this case, the effects of response-apparatus position, hand, and hand posture conform more closely to predictions of the asymmetric coding accounts than to those of the motoric accounts. We also evaluate the mechanisms proposed by the alternative accounts in terms of related literature on the properties of spatial and verbal codes. Evidence indicates that spatial information is represented in categorical and coordinate codes, and both categorical spatial codes and verbal codes are asymmetric. Experiments on mental rotation suggest that it is unlikely that the direction of rotation is determined automatically by movement constraints, as the end-state comfort hypothesis suggests. An explanation in terms of salient features and referential coding can accommodate the range of orthogonal SRC effects.