Not all toilet flappers look alike at first glance, while others look similar but have subtle differences that affect how they operate and what toilets they work inside. No matter how a flapper looks or functions, they all cover the drain hole in the bottom of the toilet's tank until you flush the toilet and open the valve to let the water out of the tank.
American Standard Actuators
American Standard used actuators in toilets before it turned to traditional flapper valves found in toilets today. An actuator performed the same function as a flapper valve, but instead of a flap an actuator has two tubes. The first tube has a rubber seal on the bottom, which seals off the tank's drain opening. The second tube sits on top of the first one, and has a chain attached to it, which also attaches to the arm in the toilet. When the water inside the top tube drains out, the actuator closes back over the drain.
Many toilets that have a flapper valve have a traditional type. The flapper itself may be made of plastic with a rubber seal on the bottom, or it may be made entirely of rubber. The flapper sits over the drain opening in the bottom of the tank, and the back side of the flapper attaches to the fill valve tube. When you press the toilet's handle, the arm in the toilet lifts and pulls on the chain attached to the flapper. The flapper floats in the water, until the water drains out of the tank and the flapper closes again.
Low flow flappers may look like traditional flappers, depending on their design, but they operate differently. A low flow flapper may have a hole in the top of the flapper, which will cause it to take on water at a certain rate. The taking on of water will affect how long the flapper stays open each time you flush the toilet, affecting how much water in the tank exits with each flush. Other low flow flappers have a float on the chain, which you adjust to whatever height you desire. Once the water level drops below the level of the float, the flapper falls closed over the drain opening.
Different brand flappers use different ways of attaching to the fill valve in the toilet. Some brands use a rubber ring on the end of the flapper, which fits around the fill valve's tube. Other brands use a tab that inserts into the fill valve, while others have ears that fit over a hinge piece on the fill valve. Some brand toilets use three inch diameter flapper valves on some models, while other models may use four inch diameter flapper valves. Taking the old flapper with you when you purchase a new one will help you purchase a matching replacement.