I get involved in two or three trailers a year.
Here, non braked or intertia braked trailers use a simple 4 pole flat plug, with Ground (white, male prong), Tail (brown), Yellow (Left Stop/Turn), Green (Right Stop/Turn)
Electric braked trailers use a 7 pin round connector (which uses flat pins), and adds continuous power, Brake (controlled from an electronic brake controller added to the tow vehicle), and an auxillary line.
Air braked trailers use yet another 7 pin standard, with separate circuits for clearance and tail lights.
