Stringers are the uprights or sides of a ladder.
Source: IMO MSC.1/Circ.1352, amendments to the Code of Safe Practice for Cargo Stowage and Securing (CSS Code), 30 June 2010, International Maritime Organization. Regulatory Guidance
.1 the frame of a ladder; or
.2 the stiffened horizontal plating structure fitted on the side shell, transverse bulkheads and/or longitudinal bulkheads in the space. For the purpose of ballast tanks of less than 5 m width forming double side spaces, the horizontal plating structure is credited as a stringer and a longitudinal permanent means of access, if it provides a continuous passage of 600 mm or more in width past frames or stiffeners on the side shell or longitudinal bulkhead. Openings in stringer plating utilized as permanent means of access shall be arranged with guard rails or grid covers to provide safe passage on the stringer or safe access to each transverse web.
Source: IMO Resolution MSC.158(78), amendments to the Technical provisions for means of access for inspections (Resolution MSC.133(76)), 20 May 2004, International Maritime Organization. Regulatory Guidance