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This is a faucet coming out of a wall in my backyard. What is that top white round part used for? What tool is used to screw/unscrew it?

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up vote 5 down vote accepted

That is a backflow preventer, which makes sure that if the water pressure in the house plumbing drops, it doesn't suck water from the hose or whatever the hose might be sitting in.

You can also find information on similar devices by searching for "vacuum breaker".

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