The first place to go is the place you get a building permit from, i.e. the place with jurisdiction over what you will build (many local libraries have copies of codes for free inspection). Each municipality usually adopts the basic code as noted above, 4", etc. but some opt for more restrictive rules. To further muddy the water, there are places with NO codes, but then there is the National code.
Know the rules before you start. Do NOT assume anything or let anyone tell you that "x" is ok! Pay for a copy of that portion of the code applicable to your project and KEEP IT!
None of this really matters until someone gets hurt!
What he says ond one thing more. If I have ANY question about a job that seems to deviate or conflict with code, I go directly to the building inspector for clarification. It may take a little more time, but there is no tearing out and re-doing later.