Just guitar and percussion--and a good deal more music than you'd expect that combination to yield. David Onderdonk ranks among the city's most creative, least-known guitarists, and he's a composer of unusual and wonderful tunes; Mark Walker's versatility and imagination have helped make him the favorite drummer of what seems to be half the city's musicians. Of course, Onderdonk and Walker alone would hardly seem to consititute a "band," but these are modern times. So Walker augments his assortment of small percussion instruments with an Octopad, allowing him to program a wide assortment of drum sounds as needed, while Onderdonk switches among three guitars--classical, steel-string, and the relatively rare eight-string Telecaster--as each song dictates. It would have been easy to take things further and bring in the computers, hook up the synths, and use sampling and sequencing to create the illusion of who knows how many performers, but this music has a different purpose: although Onderdonk and Walker can rock out (and their command of the technology allows them to avoid sonic monotony), this music never gets far from its graceful and elemental foundation. It's still about one guy banging sticks in time, and another guy shortening and plucking strings in tune, and that simplicity of focus makes this the most unobtrusively centered music on the local scene. Tuesday, Oz, 2917 N. Sheffield; 975-8100.