Ska is roughly a mixture of calypso and jazz. It is more percussive than melodic and very upbeat.
Rocksteady is the name of the genre between ska and reggae. It was intended for dance, was much slower than ska and used offbeat rhythms. Alton Ellis is known as the Godfather of Rocksteady. You'll also find bands like the Skatalites and a number of popular ska artists in this style.
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