Dr. Harris’s class takes place every Tuesday night at a rehearsal studio in Midtown. It has three segments: piano from 6 to 8, vocals from 8 to 10, and improvisation for all instruments, from 10 to midnight. Everyone is welcome, and the website notes that you don’t even know how to play piano to attend. Six hours of jazz instruction for $15.

“It’s the most beautiful thing you want to hear in your life,” Dr. Harris said of the sound of a musician whose skills improve after working with him.

Originally from Detroit, where he started teaching at age 15 out of his mother’s house, Dr. Harris moved to New York in 1960. He soon became friends with the Baroness Pannonica de Koenigswarter, a Rothschild scion and jazz patron, who invited him to move into her modern-style house, which had stunning views of the Hudson River, in Weehawken, N.J. And a hundred cats.

Thelonious Monk joined him around 1972, dubbing it the “Cat House” and staying until his death 10 years later. Dr. Harris still lives there today. The baroness died in 1988, and she made arrangements so that Dr. Harris could live there as long as he wanted.

These days, Dr. Harris’s friends drive him into the city for gigs and for the workshop.

The students — who range in age from 20 to 60 and vary widely in experience and ability — sit or stand as close to Dr. Harris as they can, watching intently. The effect is as if he were teaching in a fishbowl. Many have been coming to the workshop for decades. And they know they need to come prepared.