Chamber operas are basically small-scale, contemporary operas. Neither is the audience for them large; I hadn't even heard the term much before Pittsburgh's Microscopic Opera, dedicated to such works, appeared, in early 2010. 

But the field has its stars. One is surely composer Jake Heggie, best known his full-scale opera Dead Man Walking, which Pittsburgh Opera staged several years back. Heggie also composed the stunning chamber opera “To Hell and Back." That piece was one of two works Microscopic staged in its debut. And to Heggie the company returns with its latest winner, Three Decembers.

This chamber opera is Heggie and librettist Gene Sheer's adaptation of Terrence McNally's stage play Some Christmas Letters. Three Decembers is a beautiful piece, with gorgeous music and a good deal of depth to the characters: a stage-actress mother and the two adult children with whom she has a torturous relationship, as played out over three holiday seasons a decade apart, 1986-2006.

The live, 12-member orchestra conducted by artistic director Andres Cladera provides the music, and the cast of three the voices. Soprano Mary Gould sings Madeline Mitchell, the stage diva; baritone Daniel Teadt her son, Charlie, whose lover dying of AIDS she'll barely acknowledge; and soprano Erica Olden his sister and confidante, Beatrice.

At about 100 minutes, including an intermission, Three Decembers, directed by Lisa Ann Goldsmith, is the longest work Microscopic has staged yet. The singing is great, the material emotionally fraught but regularly leavened with humor.

The only thing that might test your patience, in this show staged in Pittsburgh Opera's Strip District studios, is making out the lyrics in Act I. At first, the voices seemed to compete with the orchestra. That's less problematic with verse-chorus songs like you'd get in musical theater, moreso when the lyrics are actually complicated dialogue, sung through. But gradually the ear adapts (or in-house adjustments were made?) and by Act II at Thursday night's opening performance, discerning both the words and the music was easy enough.

Three Decembers is the last Microscopic show to be staged while Cladera, who's also done fine work as artistic director of the Rennaissance City Choirs and with Quantum Theatre, is still a Pittsburgher. But though Cladera's moving to Colorado, he and Olden -- who's also cofounder and artistic director -- will continue to run the company.

Three performances of Three Decembers remain as of this writing: at 8 p.m. tonight and Sat., Nov. 19, and 7 p.m. Sun., Nov. 20. www.microscopicopera.org