A term used in the phonetic classification of speech sounds referring to the overall muscular effort used in producing a sound. The contrasts are labelled variously, e.g. fortis v. lenis, tense v. lax. This contrast is viewed as particularly important in distinctive feature theories of phonology, where ‘tense’ is one of the main features set up to handle variations in manner of articulation. Tense sounds have been defined both articulatorily and acoustically: they are sounds produced with a relatively strong muscular effort, involving a greater movement of the (supraglottal) vocal tract away from the positon of rest, and a relatively strong spread of acoustic energy. The opposite term in Jakobson and Halle’s system is lax; in Chomsky and Halle’s later system, the term non-tense is also used: these are sounds produced with less muscular effort and movement, and which are relatively short and indistinct, involving a relatively weak spread of acoustic energy as in centralized vowels. Subglottal tension in Chomsky and Halle’s system is handled by the feature heightened subglottal pressure.