Temporal grouping effects in verbal and spatial serial recall suggest that the representation of serial order in verbal and spatial short-term memory (STM) incorporates positional information. However, not all effects of grouping are created equal in the verbal and spatial domains. Although grouping a sequence of verbal items engenders an increase in between-group transpositions that maintain their within-group position, grouping a sequence of spatial items does not engender an increase in these so-called interposition errors. Here I present experimental and computational modeling evidence which suggests that positional information is represented in subtly different ways in verbal and spatial STM. Specifically, the findings indicate that in verbal STM, groups are coded for their position in a sequence and items are coded for their position in a group. By contrast, in spatial STM groups are coded for their position in a sequence, but items are coded for their position in a sequence, rather than in a group. Findings support the notion that positional information in verbal and spatial STM is represented by modality-specific mechanisms rather than a domain-general system.