You might have noticed a small metal circle inside a bathtub. But what is interesting about it is that this ring seems of no purpose, or that is what it looks like for most people. But in reality, that is the tub drain overflow plates. That means whenever the water level inside the tube rises above that metal circle, the valve inside it opens up and drains the excess water in the tub. It prevents the overflowing of the tubs if somebody plugs the main drain close and forgets to turn off the faucet. This valve is especially important if your flooring is wooden.