Perceptual relationships between four-voice harmonic sequences and single voices were examined in three experiments. In Experiment 1, listeners rated the extent to which single voices were musically consistent with harmonic sequences. When harmonic sequences did not change key, judgments were influenced by three sources of congruency: melody (whether the single voice was the same as the soprano voice of the harmonic sequence), chord progression (whether the single voice could be harmonized to give rise to the chord progression of the harmonic sequence), and key structure (whether or not the single voice implied modulation). When key changes occurred, sensitivity to sources of congruency was reduced. In Experiment 2, another interpretation of the results was examined: that consistency ratings were based on congruency in well-formedness. Listeners provided well-formedness ratings of the single voices and harmonic sequences. A multiple regression analysis suggested that consistency ratings were based not merely on well-formedness but on congruency in melody, chord progression, and key structure. In Experiment 3, listeners rated the extent of modulation in harmonic sequences and in each voice of the sequences. Discrimination between modulation conditions was greater for single voices than for harmonic sequences, suggesting that abstraction of key from melody may occur without reference to implied harmony. A partially hierarchical system for processing melody, harmony, and key is proposed.