What is this 2nd copper water pipe for?

[menudoboy]

For every fixture in the house (sink, toilet, wash machine, hot nozzle to the bath tub, cold nozzle to the bathtub, etc) there is a 2nd pipe on the side coming out of the slab and going into the pipe that leads to the fixture.

What is this pipe for?

Does it have a name?

What does it hook to under the slab? (My guess is they are both hooked to the same pipe under the slab but I can understand why.)

Some basic info on this plumbing: The house is from the early 1960s so it may be something they did back then but no longer do since I can find it anywhere on the net.

(Sorry for the messy drawing but didn't have access to a camera atm.)

The part of the pipe I am asking about is marked ? on drawing:

http://img37.imageshack.us/img37/1565/pipe2nd.jpg

Thanks

[LiQuId]

if its on the hot side then i'd wager a recirc line.

[menudoboy]

It's actually on all pipes, hot pipe and the cold pipe to the bath tub, the toilet, the cold pipe and the hot pipe going that the washer hooks up to, and the cold pipe that goes into the hot water heater.

So is this pipe necessary? It seems to take a good 3 minutes just to get hot water to the sinks and bath tub already.

Does it serve a purpose for being on cold pipes too?

[johnjh2o]

That is how copper water lines are run under the slap. The run from say the water heater hot and cold under the slab then up to one fixture. From that point they drop back down under the slab (that's the second line you see) the 2nd line then comes up at another fixture and so on throughout the house. In some places you could see multiple lines
connecting together. The reason for this is to eliminate joints under the slab. In other words the water line are loop from one fixture to another keeping all joints in the walls rather then under the slab.

John

[menudoboy]

Ahh okay. Now it all makes sense.

Thanks

[Another-Plumber]

its called a manifold