The chorus members make their way into the rehearsal hall and head to their favorite seats.

Purses are tucked beneath chairs. Music is settled onto stands. Some sing quietly to themselves. It’s the number of walkers that signal this is not a typical chorus.

This is the Tremble Clefs, a small East Bay chorus made up of residents with Parkinson’s disease and those affected by it. Many of the singers are accompanied by caregivers. Some are wracked by spasmodic and jerky movements. Notes held too long can become wispy and frail.

That they are here at all, singing their hearts out with unmatched joy, is nothing less than miraculous. Once they begin to sing, those differences fade to the background.

John Cornelius, 75, once lived and died with his voice. A chemist by training, Cornelius spent most of his career selling high-tech scientific tools. He often had to talk hospitals and research labs into spending millions of dollars on a piece of equipment the size of a dining-room table.

“My voice was always very important to me,” the Alamo resident said. “I had to know about the instrument, but I had to know how to sell it.”

Nine years ago, he retired; this past spring, he learned he has Parkinson’s. By that time, the voice that had ushered million-dollar deals had become a faint whisper, his body no longer able to produce the sound, his facial muscles no longer able to form the words.

More than 1.5 million in the United States have Parkinson’s, a progressive disease that attacks the central nervous system. Those with the disease lose motor skills and speech, and they can sometimes suffer from uncontrollable tremors. In essence, they lose the ability to move automatically.

In recent years, researchers have found that singing can help strengthen the voices of Parkinsonians.

That was a connection that intrigued Margy Hansell, of Walnut Creek, founder of the Tremble Clefs. The chorus was modeled after a handful of other groups.

Hansell, who was diagnosed with Parkinson’s when she was in her early 30s, wrote a grant and received $11,500 from the National Parkinson Foundation to hire a choir director and get the Tremble Clefs off the ground. That was almost four years ago.

John Cornelius admits to being initially skeptical, but his wife, Teri, convinced him to attend his first rehearsal. It was love at first note. Cornelius, who also suffers from early-stage dementia, not only has fun singing, but his voice has grown strong and his speech has improved.