From the start, Ken Vandermark conceived the Territory Band as a forum where local free-jazz musicians and European free improvisers of different generations (such as veteran percussionist Paul Lytton and younger trumpet radical Axel Dorner) could coexist without compromising their respective aesthetic imperatives. The ensemble convenes roughly once a year, and over time it's gotten larger and less American. Musically it's explored several kinds of tension--between near silence and dense sonority, between measured time and boundless abstraction, between acoustic and electronic sound--and this development has outpaced its CD-release schedule; the group's most recent album, last year's Company Switch (Okka Disk), was recorded by Territory Band 4, but the 13-piece lineup performing this week is version 6. "Collide," the piece they'll play tonight, is a concerto for local tenor saxophonist Fred Anderson that Vandermark composed especially for the occasion. Anderson's trio with bassist Harrison Bankhead and drummer Hamid Drake will open. Ad hoc small groups assembled from members of the band will perform at 9 PM tonight at the Velvet Lounge, 67 E. Cermak; call 312-791-9050. Other small groups play Sunday at the Hungry Brain, Tuesday at the Empty Bottle, and Wednesday at the Hideout; for more info see separate Treatment items, including Wednesday's capsule for the DKV Trio. Thu 8/24, 6:30 PM, Pritzker Pavilion, Millennium Park, 100 N. Michigan, 312-742-1168. Free. All ages.