In music, there are wall genres and window genres. Here's what I mean.

Jazz, for instance, is an assimilationist window genre. It tends to absorb any kind of music with which it comes into contact. The early-music wing of classical music and old-timey folk music are essentially preservationist wall genres, attempting to defend the purity of their ancient forms.

But this is not now (and never has been) a "this or that" notion, as the splendid early-music group Musica Pacifica will demonstrate when it brings a program of 16th- and 17th-century music from the British Isles to the final Music at Noon concert in Behrend's 2009-10 season.

The Tuesday afternoon concert has its moments of high art (by Henry Purcell and Francesco Veracini), to be sure, but the meat of the program is folk music arranged for the four players -- Judith Linsenberg, recorder; Elizabeth Blumenstock, violin; David Morris, viola da gamba; and Charles Sherman, harpsichord.

Some of these songs from Scotland, Ireland, and England were arranged by Blumenstock, but one piece, "A Sonata of Scots Tunes," written in 1740 by the Englishman James Oswald, is, according to Linsenberg, "a baroque trio sonata in form using traditional Scots tunes as the basis for each movement." And you thought that fiddler Mark O'Connor invented folk-classical crossover music?

The program (much of which can be viewed on the ensemble's YouTube channel ( / user / MusicaPacificaSF) is full of such surprises. And the greatest surprise of all might be how free of gimmickry these arrangements sound.

"Most songs have come down to us as single-line melodies," Linsenberg told me from the ensemble's home base in the San Francisco Bay area. "These pieces have been done for so many years in so many different ways. So we added ours."

In most cases, Musica Pacifica added a second melody line and a bass line, but the result sounds utterly natural. And why shouldn't it? The instrumentation of violin, recorder, gamba, and harpsichord is not terribly distant from the lineup of fiddle, pennywhistle, bass, and accordion or concertina one might find at a folk festival.

Necessity may be the mother of invention, but longevity has a hand in it, too.

"This will be our 20th anniversary season," Linsenberg told me. "I don't want to say that we've exhausted traditional Baroque music, but we've played a lot of it. I love the [British Isles] music, and most of it is from the same period as the high-art music we play. I wanted to incorporate it into the stuff that Musica Pacifica did."

If this sounds like the methodology of a jazz musician, you've guessed Linsenberg's latest musical pursuit.

"I'm just a baby jazz player," she admitted. "Nothing professional yet, but hopefully someday."

Though crossover projects keep the ensemble fresh (she and Morris are heard on alt-folkie Joanna Newsom's latest CD, "Have One on Me"), there is plenty of early music to be played -- and recorded.

The program of music for Tuesday's concert is being prepared for CD release by the early-music ensemble that is more interested in building windows than walls.