Automotive radiators use a thin foil material, rolled formed into a corrugated pattern, to transfer heat from the cooling tubes to the air. During the roll forming process small windows or louvers are stamped into the foil. The louvers are formed in a group on one half of the material and then a mirror opposite on the other. As air passes through the radiator core, the louvers deflect the air flow in a S-type or snake pattern. As a result, the term "serpentine" was adopted. Louver pattern, angle, and width can all have a specific effect on heat transfer.