Stratigraphic interpretation rests on two principles: the principle of superposition and the principle that deposition units can be identified by cultural content. The sequence of deposition units derived from a case of superposition may not give a true cultural sequence if mixing, filling, or collecting has affected the cultural contents of the units. There are two kinds of seriation: evolutionary seriation, done on the basis of an assumed general law of cultural development, and similary seriation, done on the basis of similarities and differences in objects or deposition units compared. Similiary seriation assumes only that cultural change is normally gradual. Of the two kinds of seriation, only similiary seriation can give credible results. Some evidence of archaeological associations is necessary to control the possibility of non-gradual change resulting from sudden outside influence or archaism. If the conditions for success can be met, either stratigraphy or seriation can provide a credible sequence. Each method provides a check on the other; the most credible results are achieved by combining the two.