Wide mouth pint jars have straight sides and are 3" internal diameter. Whether you call it a Koozie, a Kozie, a Coozy, a Cozy, or just an insulating sleeve, it's perfect for protecting your glass jar from breakage and your hand from cold or heat. Made of 100% silicone rubber, these sleeves are dishwasher, freezer, and microwave safe! Silicone is a safe, green, non-plastic (BPA free) material for food use that does not leach any chemicals. These are designed for current production Ball jars - Kerr jars are quite tight. I drink my coffee from a wide mouth pint jar every morning. My silicone sleeve keeps my hands safe, and if I need to leave the house I screw on a wide mouth silicone drinking lid. Use a different color for each family member, and stop wondering which beverage cup is yours! Microwave safe means you can heat up your beverage without removing the sleeve. Or take out a frozen jar of food, put a koozie on it, and heat it up. Since they don't touch your beverage, I often just rinse them, but they are also dishwasher safe. You can leave them on the jar, but I find that water gets trapped so I prefer to separate them to clean the jar. You could also use one to block sunlight or prying eyes from the contents of your jar. Great for kids and adults!