good catch tkanzler.......I think you mean ID lights, the three red lights centered on the rear, spaced evenly from 6" to 12", and front and rear clearance lights, at the widest part (rear clearance lights are part of the 'over 80' stop/turn/tail lights, but you don't have to use those if rear clearance lights are separate). Side markers are required, amber front and red rear, regardless of length or width, and trailers over 30' long require mid-point amber side markers on each side, too. At least that's what the US and Canadian DOTs (or NHTSA) say.
As an aside, marker lights that are PC rated can be mounted at a 45 degree angle, and serve as both clearance and side marker lamps, since they have a 180 degree arc. Lotta new trailers do this, as it eliminates a couple of lights on each side, which of course is cheaper for the manufacturer, and takes less wiring and labor.
regards
woody