A chimney is both a feature (a vertical or near-vertical crack wide enough to fit the climber's body) and a technique (using opposing pressure inside that crack to ascend). The climber wedges themselves between the two walls — typically with the back pressed against one wall, feet flat on the opposite wall, and knees slightly bent — then "walks" up by alternately moving feet and back.
Chimneying is exhausting but unique to wide cracks. The body uses opposing forces to stick in place: friction holds the back, the feet push, and the legs propel upward. Hands play a smaller role than usual — they pat the walls for stability and occasionally find holds, but the legs do most of the work. Chimneying is more about endurance and trust than finger strength.
Classic chimney climbs include some of Yosemite's offwidth routes, the splitter chimneys of Indian Creek, and the narrow trough cracks of the Welsh slate quarries. Most modern indoor gyms have at least one chimney feature for variety. Wear long pants and a long-sleeve shirt — chimneys eat skin.