The meaning of the Akkadian term ṭuppi has been hotly debated by Assyriologists for the greater part of a century. The present article argues that ṭuppi, commonly found in temporal expressions, can only refer to a one-year period. This proposal arises out of the observation that, among a substantial corpus of Neo-Babylonian house rental contracts, there is not a single one that refers explicitly to a one-year lease period. Since rental periods of relatively short duration are otherwise rather common, this unexpected gap in the data is difficult to explain unless the expression adi ṭuppišu (and variants) is understood to mean ‘for a one-year period’. The same principle applies to other categories of document which involve a finite term, such as apprenticeship contracts. The article discusses the implications of this finding for our understanding of the contents of a wide range of everyday legal and administrative documents and letters, some of which provide circumstantial evidence in support of the proposed interpretation. It also examines the historical texts in which ṭuppi occurs, namely, the Assyrian King List and the 8th century inscription of Ninurta-kudurrī-uṣur, governor of Mari and Sūḫu. It is argued that the suffix -šu in ṭuppišu is redundant, a conclusion which affects the interpretation of these historical passages since the -šu has sometimes been understood to refer to an antecedent other than the ruler in question. The implications of the proposed understanding of ṭuppišu for Mesopotamian chronology are not so very great since the consensus has tended towards assigning a period of between 0 and 2 years for the ṭuppišu reigns in AKL. Nevertheless, this interpretation of ṭuppišu as a one-year reign/period of office clarifies some hitherto rather enigmatic passages in these historical texts.