Burets (or burettes) are used in quantitative analysis to precisely measure the volume of a fluid. They are essentially graduated cylinders with a valve at one end that allows the fluid to be released very slowly and precisely from the buret so that volume changes may be recorded. Burets are graduated from the top down (i.e., 0 is at the top).
Burets designed for dispensing liquids have their valve (also called a stopcock) at the bottom. The volume of liquid dispensed from the buret is found by comparing the initial and final volumes indicated by the graduations on the buret during an experimental trial.
Gas burets are designed so that a gas generated in a chemical reaction displaces liquid from the top of the buret down. The volume of gas produced is indicated by the amount of liquid displaced. The stopcock for gas burets is located at the top so that gas may be released between experimental trials, thereby resetting the initial volume of the buret to zero.