In the city you’ve seen these convenient two-headed pieces of street furniture sticking out of buildings or the sidewalk, and perhaps even sat on them from time to time:
So what exactly are they? It depends, at least in New York City, on what color they’re painted. You’ll see them in red, yellow or green here, they only exist in front of buildings that are taller than 75 feet/six stories, and all of them feature Y-shaped “Siamese” connections.
The red one connects to the building’s standpipe system. A standpipe is that red vertical pipe that you’ll often see left exposed in the stairwell of a building, and it supplies water to hose connections on each floor.
When firefighters arrive on scene, they’ll plug a hose from the pumper truck into the standpipe connection. They can either pump water directly from the truck’s 500-gallon tank, or connect a second hose from the truck to a hydrant; either way it goes through the pumper truck so that firefighters can modulate the pressure with the truck’s on-board controls.
The green one connects to the building’s sprinkler system.
And finally, the yellow ones are for buildings that have the standpipe and sprinkler systems connected.
All of the connections are Siamese, i.e. doubled, in case more water/pressure is required. Having two connections also serves as insurance; if the threads are damaged on one connection, the hose can be attached to the other.
So now you know.
http://www.core77.com/posts/74287/Todays-Urban-Design-Observation-What-Exactly-are-These-Pieces-of-Street-Furniture-and-How-Do-They-Help-Firefighters