Mason jar

A Mason jar is a molded glass jar used in home canning to preserve food. The mouth of the jar has screw threads on its outer perimeter to accept a metal ring (or "band"). The band, when screwed down, presses a separate stamped steel disc-shaped lid against the rim of the jar. An integral rubber ring on the underside of the lid creates a hermetic seal to the jar. The bands and lids usually come with new jars, and bands and lids are also sold separately; while the bands are reusable, the lids are intended for single use when canning.