To those who’ve never heard an authentic jazz tap dancer, the idea seems too preposterous to be on the level. (And don’t bother checking the date–April 1 was weeks ago.) But arcane as it may be, jazz-tap has a long history, enlivened by such virtuosos as Baby Laurence, Honey Coles, Jimmy Slyde, and even Fred Astaire, who recorded several tunes using just his feet in the 1950s (reissued on the DRG label). That’s the essence: just the feet. Jazz-tap artists place a microphone near the dancing surface and provide an ongoing percussion commentary, usually punctuated by brief solos, to their musical accompaniment. What separates their improvisations from more traditional percussionists and drummers is the brittle clarity of the sound, due in no small part to the enormous pressure exerted by the human foot. You can see–and more to the point, hear–jazz-tap in action when Sarah Petronio joins the Green Mill All-Stars tonight, for several tunes each set. Petronio, who became a protege and dancing partner of Jimmy Slyde during the 20 years she lived in Paris, long ago moved past jazz-tap’s expected musical base, the easy swing rhythms of Count Basie and his musical progeny; instead, she prefers to apply her feet to music by more modern artists, such as Thelonious Monk, Bill Evans, and Antonio Carlos Jobim. But she solos with the straightforward delight in pure motion that characterized Basie’s greatest drummer, Jo Jones. Tonight, 8 PM, Green Mill, 4802 N. Broadway; 878-5552.