In Japanese water basin is called Tsukubai. They are usually placed in tea garden or temple garden. Zen gardens do not always have the basin.
Composition of stone basin
The correct tsukubai is composed of three stones.
One is basin which we call chou-zu-bachi in Japanese.
Maei-ishi is placed on the right. This is step stone.
Yuoke-ishi is placed on the left. This is the place to put a kettle.
To write this, I checked Wilipedia. It tells different things. I realised that the correct way placing stone depends on the denomination.
The place of purify
Water basin is the place to purify. Guests rinse their hands and mouth.