Most boats need to have a port (red) and a starboard (green) side light each showing an unbroken light over an arc of the horizon of 112.5°. If the design of the boat allows, a combination port and starboard light unit can be mounted on the centreline of the boat, in place of two individual side lights. Individual side lights come in two styles, those intended to be mounted on a horizontal surface such as a deck and those intended to be mounted on a vertical surface such as the topsides or the side of the cabin. Be careful not to mount lights on a horizontal surface if they are designed to be mounted on a vertical surface, and vice-versa, because they will shine in the wrong direction.
Horizontally mounted side lights generally come with a reference line marked on them which must be kept parallel to the centreline of the boat when fitting the light.