Build your own string light and buy your cord and bulbs separately. Cords are available in three socket sizes — Medium/E26, C9/E17 and C7/E12. • Medium base — heavy-duty commercial grade cords with weatherproof sockets. • C9 base — commercial grade cords with weatherproof sockets, as well as standard grade cords for indoor or outdoor string lighting. • C7 base — commercial grade bulk reels and inexpensive standard grade cord options for all-purpose string lighting use.