The Wailin' Jennys, "Fifteen" (Red House Records)

It's easy to make a terrific album, as the Wailin' Jennys show on "Fifteen." Their formula: three marvelous voices blending beautifully to sing a batch of great songs. Simple, right?

Heather Masse, Nicky Mehta and Ruth Moody wisely don't let anything else get in the way. Sparse, tasteful accompaniment on the nine tunes leaves lots of room for their wonderfully intricate, silky vocal arrangements.

The three Ms swap the lead often, like a band of female Wilburys. They sing duets, call and response, rounds — talk about roots music — and in unison. When they stack three-part harmonies, they take turns changing notes, each time subtly altering the chord. It's melismatic magic.

Jane Siberry's ballad "The Valley" unfolds slowly and is gorgeous times three. Other highlights include the gospel opener "Old Churchyard," Emmylou Harris' best composition, "Boulder to Birmingham," and Tom Petty's "Wildflowers." Hopefully he had a chance to hear it.

The Jennys save their sassiest performance for the a cappella finale "Weary Blues from Waitin'," scooping and sliding with glee to make the Hank Williams obscurity sound more like something by Stephen Foster. They linger on the final word, climbing an octave to heavenly.