Most chalk bags have a chain and zipper system. It can be opened and closed quickly with the chain. Some chalk bags may have straps which shut it close. Some also may open and close with the pull of a string.
Most climbers prefer to use chalk bags that have chains on them as it helps to secure the chalk inside properly and prevents spillage. Some modern chalk bags have an opening that need not be closed, and the climber simply puts his hand inside to grab the chalk. An inside layer of stitching in the bad helps to prevent the chalk from spilling out.