In this study, we report a corpus analysis of rock harmony. As a corpus, we used Rolling Stone magazine's list of the ‘500 Greatest Songs of All Time’; we took the 20 top-ranked songs from each decade (the 1950s through the 1990s), creating a set of 100 songs. Both authors analysed all 100 songs by hand, using conventional Roman numeral symbols. Agreement between the two sets of analyses was over 90 per cent. The analyses were encoded using a recursive notation, similar to a context-free grammar, allowing repeating sections to be encoded succinctly. The aggregate data was then subjected to a variety of statistical analyses. We examined the frequency of different chords and chord transitions. The results showed that IV is the most common chord after I and is especially common preceding the tonic. Other results concern the frequency of different root motions, patterns of co-occurrence between chords, and changes in harmonic practice across time.