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 both transparent and opaque vowels are not common; Lokaa has both. Secondly, though Lokaa has an eight-vowel inventory the vowels [a] and [ə] have not “re-paired”. Thirdly, the historic ATR contrast found in Benue-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 merger of high vowels in Lokaa is not complete. I show that the analytic framework of “headed span” theory accounts for a system with both transparent and opaque vowels. More importantly, I propose that within the headed span approach to harmony, both co-occurrence constraints and ASSOCIATEHEAD are crucial. Co-occurrence constraints control the language inventory, and the different rankings of the ASSOCIATEHEAD constraints indicate whether or not a feature will form part of an harmonic span.