Three solutions to the problem of serial order can be identified: chaining, ordinal and positional theories. Error patterns in serial recall from short-term memory fail to support chaining theories, yet provide unequivocal evidence for positional theories. In a new model of short-term memory, the Start-End Model (SEM), the positions of items in a sequence are coded relative to the start and end of that sequence. Simulations confirm SEM's ability to capture the main phenomena in serial recall, such as the effects of primacy, recency, list length, grouping, modality, redundant suffices, proactive interference, retention interval, and phonological similarity. Moreover, SEM is the first model to capture the complete pattern of errors, including transpositions, repetitions, omissions, intrusions, confusions, and, in particular, positional errors between groups and between trials. Unlike other positional models however, SEM predicts that positional errors will maintain relative rather than absolute position, in agreement with recent experiments (Henson, 1977).