The canonical image of vowel harmony is of a particular feature distributed throughout a word, leading to symmetric constraints like A GREE or S PREAD . Examination of the distribution of tongue-root advancement in Kinande demonstrates that harmonic feature distribution is asymmetric. The data argue that a formal (yet asymmetric) constraint (like A LIGN ) is exactly half right: such a constraint correctly characterises the left edge of the harmonic domain. By contrast, the right edge is necessarily characterised by phonetically grounded restrictions on feature co-occurrence. Of further interest is the role of morphological domains: the interaction between domain restrictions on specific constraints and unrestricted constraints suggests a formal means of characterising the overwhelming similarity between constraint hierarchies at different morphological levels while at the same time characterising the distinctions between levels.