Deictic relations specify a relation in terms of the perspective of the speaker such as left/right; I/you (and all of its correlates, such as "mine"; here/there; and now/then. Some relations may or may not be deictic, such as front/back or above/below, depending on the perspective applied. For example, the sentence "The back door of my house is in front of me" contains both a spatial and deictic form of "front/back." Deictic relations seem to be a particularly important family of relational frames that may be critical for perspective-taking. An example is the three frames of I and YOU, HERE and THERE, and NOW and THEN. These frames are unlike the others mentioned previously in that they do not appear to have any formal or nonarbitrary counterparts. Coordination, for instance, is based on formal identity or sameness, and "bigger than" is based on relative size. In contrast, frames that depend on perspective cannot be traced to formal dimensions in the environment at all; instead, the relationship between the individual and other events serves as the constant variable upon which these frames are based.