This is the only toilet flapper with built-in Microban protection for use in 3.5 gallons per flush toilets made before 1994. The Microban protection keeps the flapper cleaner by fighting flapper breakdown due to bacteria in the toilet tank. This Universal Longest Lasting Toilet Flapper has a solid frame for ultimate reliability. Flappers with a solid frame do not twist and provide the best seal. Built-in Microban fights flapper breakdown due to bacteria. Chlorine and Hard Water Resistant Solid Frame For Ultimate Reliability. Easy to install- works on most toilets.