Both dome lid and base (combo pack) are made with microwavable polypropylene. Lids are easily secured and will not crack like those made of polystyrene. Leak-resistant lid fit eliminates the need to over wrap with film or tape. Double vents allow steam to escape more freely to reduce condensation buildup. The unique stacking feature keeps the container secure without blocking the vents in the lid. The Revolutionary V-lock system prevents moisture from collecting around the flange to eliminate leakage and improve lid fit.