Bathroom Sink Overflow Hole

Where is a sink overflow hole placed?
Sink overflow hole, or sink drainage opening, is often located near the top of a sink at which some people call the high water mark in the sink. It is at the top of the sink overflow channel, which is often hidden and normally cannot be seen. For a sink, there can be one or two sink overflow holes.
 
What functions does an overflow hole have?
In general, there are two functions of a sink overflow hole.
 
At first, the sink overflow prevents water from overflowing. Sometimes, you might leave the faucet on and keep the drain closed at the same time. When the running water reaches the sink overflow hole, the water will drain out through the overflow hole. It prevents the water overflowing from your bathroom sink to wet the floor.
 
The other function of a sink overflow hole is to improve the drainage. With an overflow hole on the sink, more air is allowed into the drain pipe, thus helping the sink drain faster. In daily life, we can discover that bathroom sinks with overflow holes usually drain faster than those without overflow holes. It has something with the SUCTION EFFECT.