Fortition is well-known to target initial prosodic edges while lenition occurs in medial and final positions. I argue that lenition processes can be used to diagnose whether a pair of syllables is parsed together in a foot just as fortition can be used to locate an initial foot boundary. Hidatsa (Siouan, North Dakota) is a quantity-sensitive iambic language (Metzler 2021) in which stops and affricates become voiced intervocalically (Harris & Voegelin 1939). The duration of voicing of intervocalically voiced stops is sensitive to foot structure: the duration of voicing in intervocalic stops is greater foot-medially than in other prosodic environments. Although the prominent voicing characteristic of foot-medial stops is found in pairs of light syllables (LL), it is absent in a light-heavy syllable pair (LH). This suggest that asymmetrical iambs (LH) are not permissible groupings, posing a challenge to the longstanding view that asymmetrical iambs are not only grammatical, but the optimal iamb (Hayes 1995).
