It is argued that the encoding of conceptual categories such as ‘realis’ should be investigated by analyzing the meanings of linguistic signs rather than by focusing on a priori message-categories that may not be expressed by specific linguistic forms. Applying this method to the expression of ‘realis’ in Swahili reveals a complex relationship between tense-aspect-modality, negation, and realis status in which some forms signal information related to irrealis, but no forms explicitly signal the meaning ‘realis’; the two negation markers span the realis/irrealis divide. Sign-based analysis makes it possible to identify subtle differences between apparently synonymous messages that are conveyed by forms differing in realis status.