This study argues for a reformulation of the semiotic and characteristically linguistic construct of Duality of Patterning. Since the Duality principle was originally formulated, linguists have generally accepted the ‘barrier’ enshrined in Duality, said to separate the levels of Expression (in particular, its Phonological component) and Wording (Vocabulary and Grammar). However, there was always criticism of the strong formulations of Duality, some from precursors of functional models of language, notably Systemic-Functional Grammar (SFG). On the other hand, although SFG has made a rich and original contribution to the understanding of intonation systems and has controversially defended a ‘natural’, permeable relation between Wording and Meaning, it has allowed the principle of Duality to be treated uncontroversially. There are also a number of flourishing misunderstandings about Duality, which this study will explain and rectify: its alleged bond with Arbitrariness and with ‘meaningless’ phonemes rather than contrastive phonetic features. In this study, Duality is characterised as a permeable relation rather than a strict barrier. Much evidence in natural languages and in the literature supports this less rigid view of Duality: Pike, Jakobson, Firth and, more recently, Halliday (1992) following Lemke (1984).