The Lexington Chamber Chorale's Tuesday night rehearsal starts with a rendition of Aftonen by Swedish composer Hugo Alfvén.
It is measured, controlled and delicately shaped by conductor Gary Anderson. It's what you want from a choral work.
Then, for the next piece, Anderson delivers a direction that most choral singers will probably never hear.
It starts as a standard warm-up scale, but then he instructs the 30-person choir to sing any notes they want in the key of C sharp.
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"If you want to go by thirds, fourths, whatever," he tells the singers. "Keep it diatonic, and I'll meet you at the end."
That warm-up quickly bursts into a constellation of notes filling the practice room in Transylvania's Mitchell Fine Arts Building.
"It's choral improvisation," Anderson says during a chat before the rehearsal. "You do anything you want within the template of a scale. It depends on the independence of a singer."
And it is something most choral singers are never trained to do, Anderson says.
He was trained in the technique by conductor Gary Graden of Stockholm, Sweden, who had been scheduled to come to Lexington to conduct this weekend's concert of mostly Swedish music. Graden had to cancel his visit because he has pneumonia.
"He has been e-mailing me constantly, saying, 'This is what I'd do here, this is what I'd do here," says Anderson, who will now conduct Saturday's concert.
Choral improvisation has its roots in jazz, Anderson says, as does much musical improvisation. Like jazz, Anderson said, it takes a fairly high level of skill to pull it off.
He thinks his singers are ready.
And the performance comes when the 19-year-old choir's star is rising in the local arts community. Last month, the Lexington Philharmonic Orchestra announced the Chamber Chorale will be the chorus for its annual production of George Frideric Handel's Messiah, which will be presented in a Baroque style at Cathedral of Christ the King.
"It is easily the biggest thing we have done," Anderson says.
Virginia Bowles, a charter Chamber Chorale member, says, "It is a great opportunity for us and will really focus our energy."
For now, the choir's energy is focused on a completely new style of singing, in which they are given a key and set free. Tuesday evening, they also practiced in a formation in which the voices are mixed standing around the audience.
"It literally is Surround Sound," Anderson says to the chorale.
Bowles likes the challenge: "You never know what's going to happen. Just stay alert to what's going on around you and stay alert to your own part."