Golden Harvest Regular Mouth Mason Jar Lids in a 12 Pack box are the lids you trust. Golden Harvest Regular Mouth Lids for preserving fit all regular mouth glass preserving jars. Seal in the freshness and enjoy freshly made foods tonight or tomorrow. Lids are for one-time canning use only. After the first use, the lid will no longer seal effectively, so a fresh lid must be used every time. Pre-heating lids is not required! After extensive testing by Golden Harvest's Quality Assurance Team, they determined that it is no longer necessary to pre-warm lids before use. If you desire, it is still safe to simmer your lids before use, however, you should never boil them. Golden Harvest's recommendation for over 20 years has always been to simmer (180°F), not boil (212°F), the lids.