National Electrical Code Articles and Information

1. All the switching must be done only in the ungrounded conductor [404.2].

2. It's about where the grounded conductor for the controlled lighting circuit must be. Except for the seven applications listed, it must be at the location where the switches control the lighting loads that are supplied by a general-purpose branch circuit. An example exclusion is where the switch does not serve a habitable room or bathroom, and that really does cover a vast number of situations!

3. The switch must be installed such that gravity will not tend to close it [404.6(A)]. That's why in the opening remarks you read about up for on and down for off. That said, there are switches approved for use in the inverted position. But they must have an integral mechanical means of ensuring the blades stay open when set open.
   
   

4. You can't group or gang snap switches in enclosures with other snap switches, receptacle, or similar devices unless they are arranged so the voltage between adjacent devices does not exceed 300V [404.8(B)].
   
5. A faceplate must completely cover the opening. Also, where the switch is flush-mounted, the faceplate must seat against the finished surface [404.9(A)]. DIY folks and even some electricians often muff this one. The problem is the box is not properly installed. If it sits back too far, they tighten down the plate mounting screw(s) so much they bend or even crack the plate. Or maybe they leave the switch mostly recessed back behind the cover. If the box sits out too far, they also overtigten in an attempt to force it toward the wall but typically must leave a gap anyhow.
   
   So when mounting a box, use the guide marks on the box to get it properly positioned in all dimensions. If the box is already mounted and the wall is already finished, you'll have to adjust the switch tangs and/or use special adapters that are sold for fixing bone-headed box installations that put the switch(es) in the wrong place in relation to the finished surface.