The three human at-rest postures of sitting, standing, and lying are basic, recurring features of human behavior and may reasonably be called primary postures. The three postures share the property of being stable through time, but they are also differentiated in terms of their overall shape, their physiological properties, and typical associated behaviors such as the association of sitting with social interaction, and lying with sleeping. The experiential realities of the three postures underlie and motivate a range of cross-linguistic phenomena involving morphemes with meanings of “sit”, “stand,” and “lie”. The relevant linguistic phenomena include higher frequencies of occurrence compared with other kinds of posture verbs and differential behavior with respect to some morphosyntactic patterns involving notions such as agentivity. The posture morphemes can also be the source for a variety of semantic extensions reflecting experiential realities of the postures, such as the extension of “lie” to mean “sleep” in some languages. Extensions also include grammaticalizations of the posture morphemes to locative and aspectual markers which reflect the temporal stability and spatial fixedness of the postures themselves.