Paul, there are different "standard" plumbing codes in the USA and like with the NEC, different states/counties/cities can have amendments to the different "standard" codes.
Venting varies with the different codes. The UPC(Uniform Plumbing Code) requires every fixture to have a vent. The vent can tie back into a main vent stack, run independently or connect with another vent, to run up past the roof.
Some codes allow "wet venting", using a drain pipe as a vent pipe for another fixture, with little restriction, while the UPC allows wet venting on vertical pipes only with other restrictions.
steve
