Considered Bach’s musical version of a last will and testament, the Mass in B minor — which Seattle Pro Musica will perform at St. James Cathedral — may be the ultimate choral experience.

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Many classical-music fans consider Johann Sebastian Bach’s Mass in B minor as the ultimate peak of Western choral music — but the composer never heard it performed in its entirety.

“Bach didn’t write it for practical performance,” said Karen P. Thomas, artistic director of the choral group Seattle Pro Musica. “It’s too long to fit into a normal Mass setting. He compiled it when he was approaching the end of his life, using other works he’d written in the past.”

As far as we know, she explained, “Bach seems to have planned it as a compendium looking back over what he had achieved — like he did with the (unfinished) ‘Art of Fugue.’ ”

Concert preview

Seattle Pro Musica: Bach’s Mass in B minor

8 p.m. Friday-Saturday, May 13-14, at St. James Cathedral, 804 Ninth Ave., Seattle; $12-$45 (800-838-3006 or seattlepromusica.org).

This Saturday, May 14, Thomas, who composes as well as conducts, will lead her singers and a period-instrument orchestra in a performance of the monumental Mass at St. James Cathedral.

Its impracticality for liturgical use is just one of the mysteries surrounding the work — as well as the fact that Bach’s own family identified it as a Roman Catholic Mass.

The devoutly Protestant Bach also wrote a handful of shorter Latin Mass settings (during his era, Lutheran tradition still allowed the language of Rome to be sung in this context), but his turn to the Catholic model is striking.

Some scholars argue that Bach set the first two movements as a kind of job application for an appointment at the sumptuous, Catholic court in Dresden, but later returned to complete it — not for any practical purpose, but as an encyclopedic survey of the history of Western music up to his time. The result is a complex labyrinth that sets the Mass in B minor apart from Bach’s other sacred music.

Even the massive “St. Matthew Passion” was clearly written for liturgical use, just like the cycles of weekly cantatas Bach produced as part of his day job as music director of St. Thomas Church in Leipzig.

To meet the challenges posed by the B-minor Mass, Seattle Pro Musica is collaborating with the early-music organization Pacific MusicWorks, which will be led by renowned violinist Tekla Cunningham. They will also be joined by an impressive group of four vocal soloists from around the country and the Baroque trumpet virtuoso Kris Kwapis.

One advantage of working with an ensemble that specializes in Baroque music, Thomas said, is that “whenever they perform with a chorus, the instrumentalists are completely tuned in to the singers, to how they shape a phrase. They understand the role of the text and don’t just ‘play’ the notes the way they look on the page. Choral singers don’t often encounter that level of communication as a two-way street.”

Just as staging Shakespeare requires choices about textual editions, any performance of the Mass in B minor requires decisions about the number of musicians, as well as style.

Debates about the “authentic” way to perform Bach continue to rage, and the period-performance aesthetic is too multifaceted to reduce to a single approach. At one extreme, scholars argue that only one singer should be assigned per part for the choral music. In practice, Thomas said, a one-voice-per-part approach would be absurdly taxing for singers in a work as vast as the B-minor Mass.

But there are advantages to a period-performance approach — overall, Thomas said, “the sound of Bach played with period instruments makes so much more sense than when I hear it with modern instruments.”

For the Mass, she has opted for a blend of Baroque, period-instrument orchestra with a choral force of 80 singers from Seattle Pro Musica, along with four soloists. “It’s a big group, but I’ve trained the singers to approach this like chamber music,” Thomas said. “They sing with extreme clarity and lightness, so that 15 people can sound like three.”

Like so many other Bach masterpieces, the B-minor Mass had to wait until well after the composer’s death to acquire its lofty reputation. But Thomas has no doubts about its value.

“It absolutely is my desert-island piece,” she said. “Even though I have conducted it a number of times, I could study it for a lifetime and still discover new riches on every page. It’s a marvelous score on so many levels — emotional, intellectual, compositional.”