For the last 40 years, jazz guitarist and composer George Cotsirilos has been leading a double life. By day, he was a lawyer and a professor of law at UC Berkeley’s law school. When the sun went down, he picked up his guitar and played jazz, filling the night with free flowing, melodic improvisations.

“My secret lives weren’t premeditated or rigid,” Cotsirilos explained from his Berkeley home. “I just never told the law people I was a jazz musician, or told the folks in the jazz world that I was a lawyer. Oftentimes, people found out about my separate lives, but I didn’t consciously broadcast it for a couple of reasons. If musicians knew about my law practice, I had the concern of being written off as a dilettante. If I was with a client, or teaching a class, I wanted people to know I was doing my best for them. I never denied to another lawyer that I played jazz, but I didn’t go out of my way to inform them.”

When he retired from Cal in 2017, he was finally free to become a full-time musician.

Cotsirilos was born in Chicago and started classical violin lessons in grammar school. An uncle who played drums with the Woody Herman Orchestra got him interested in jazz. “He took me to hear Louis Armstrong. When he took me to see Oscar Peterson and Erroll Garner, I switched to piano.”

After seeing the Butterfield Blues Band’s jazzy approach to the blues, Cotsirilos picked up the guitar. When he came to Berkeley to study sociology, Cotsirilos brought his guitar. He went to school during the day and played in bands at night. After graduation, he toured nationally with The Whispers, a vocal group in Oakland. On his return to the Bay Area, he was pulled in two directions. “My father was a lawyer, and I was raised with a social and political consciousness. I went back to school, got a law degree, and became a deputy public defender in 1978. I started teaching at Berkeley in 1999.”

The stability of his teaching job gave Cotsirilos the freedom to play, compose, and record the sounds he was hearing in his head. He made a solo record and three albums with his trio — On the Rebop (2006), Past Present (2010), and Variations (2013). Now that he’s retired, he’s ready to go deeper into the music. On his latest album, Mostly in Blue, he brought in pianist Keith Saunders to compleºment his long-time cohorts, bassist Robb Fisher and drummer Ron Marabuto.

“The addition of a piano was dictated by the tunes I was writing,” Cotsirilos said. “Given the nature of the pieces, I wanted a little richer harmonic foundation, which also allowed me a bit of freedom to take a little different, more legato, single note approach. We had an introductory four-hour session and went through the tunes to see if we’d work as a quartet, and we clicked. We cut the album in one day, one or two takes of each tune. We didn’t stop and listen to the takes. I wanted to get as much in the can as possible. When I finally did listen, it pleasantly confirmed my feeling that we’d gotten what we needed.”

Mostly in Blue has a bluesy feeling, even though the compositions are not blues based. Cotsirilos writes melodies that leave room for the quartet members to voice their own improvisational skills. “There’s space in every song for solos. I want to take advantage of the band’s musical prowess.”

In addition to his original tunes, Blue includes two standards, Charlie Parker’s “Crazeology,” a hard swinging bop tune, and “I Wish I Knew,” a classic from the American Songbook. “The arrangements of those two pieces were spontaneous. After I knew the ensemble was working, I wanted the musicians to do what they felt on those tunes. I think that approach is much of what jazz is about.”

With his teaching behind him, Cotsirilos is pursuing music as his main gig. “We’re a local band right now, but I’m trying to line up out of town jobs. The problem is with the booking agent, who is me. I’m working on it, but I’m trying to be a band leader, a composer, and a musician. There’s so much to learn. I know I’ll go to my grave not having done a tenth of what I want to be able to do.”

The George Cotsirilos Quartet will play at 8 p.m., Wed., June 30, at The Back Room, 1984 Bonita, Berkeley, 510-654-3808, BackRoomMusic.com