A climbing chalk bag is a small cylindrical or trapezoidal fabric pouch — typically 12 to 20 cm tall — that holds gymnastic chalk (magnesium carbonate). Climbers dip their hands into it between moves to absorb sweat and improve grip on the rock or holds. The interior is usually a soft fleece lining that holds chalk without spilling, with a stiff rim and drawcord closure.
Chalk bags split into two main types. Roped-climbing chalk bags hang from a thin nylon belt around the waist and stay with the climber on the route. Bouldering chalk buckets are larger, sit on the ground beside the climb, and hold more chalk plus a brush and tape — used between attempts on a problem rather than during.
Most gyms allow loose chalk; some require chalk balls (chalk in a fabric mesh) to reduce dust. Liquid chalk — alcohol-based chalk paste that dries to a powder coating — is increasingly popular for both indoor and competition use because it produces no dust at all. When buying your first chalk bag, prioritise size (big enough for your full hand), a stable closure, and a comfortable belt. Brand and aesthetics matter little compared to function.