This paper discusses the four neutral vowels in Lokaa harmony, [i, u, ə,a]. Crosslinguistically, neutral segments are either transparent or opaque.Lokaa harmony is important in three crucial respects. First, languages with bothtransparent and opaque vowels are not common; Lokaa has both. Secondly, thoughLokaa has an eight-vowel inventory the vowels [a] and [ə] have not“re-paired”. Thirdly, the historic ATR contrast found inBenue-Congo high vowels shows up when high vowel stems take mid-vowel prefixes,though the high vowels can only be [+ATR] on the surface; the ATR mergerof high vowels in Lokaa is not complete. I show that the analytic framework of“headed span” theory accounts for a system with both transparentand opaque vowels. More importantly, I propose that within the headed spanapproach to harmony, both co-occurrence constraints and ASSOCIATEHEAD arecrucial. Co-occurrence constraints control the language inventory, and thedifferent rankings of the ASSOCIATEHEAD constraints indicate whether or not afeature will form part of an harmonic span.