"Did you know that swine changes his guitar strings once a week? Why it's not sporting," said Dave Van Ronk, referring to John Renbourn, the British guitarist who opened for him at the Birchmere last night.

Though they have a good deal in common--both are burly men with rough-hewn voices and deservedly strong reputations as guitarists--the juxtaposition of Renbourn's Celtic ballads and medieval dance tunes and Van Ronk's ragtime and blues made for some delightful contrasts--all kidding aside.

Graceful and subtle if not always perfectly precise, Renbourn's intricate arrangements of traditional material, some of it dating back to the Renaissance, won him an unusually long ovation. A sprightly transcription of a piece by the fabled Irish harpist Sean O'Carolan and a breezy Caribbean flavor of Joseph Spence's "Great Dream of Heaven" were among the highlights, colored by altered tunings and wonderful embellishments.

One of the first pieces Van Ronk sang was a tribute to the late bluesman Mississippi John Hurt. It was a fitting choice, for like Hurt, Van Ronk often plays bright, engaging melodies on the treble strings and offsets them with a sly alternating bass. And if he's not quite the guitarist Renbourn is, he is twice the singer. His ceaselessly brutal version of "St. James Infirmary" reminds one of Louis Armstrong's voice at times: raw, yet compelling in its own way.