(nautical,uncountable) Therope or similar material used to make cross-ropes on a ship.[from 14th c.]
(nautical) Any of the cross ropes between theshrouds, which form a net-likeropework, allowingsailors to climb up towards the top of themast.[from 17th c.]
1808–10,William Hickey,Memoirs of a Georgian Rake, Folio Society 1995, p. 80:
[H]e laid hold of the firstratline with his right hand, then sprung to the second, with his left, and so on alternately, right and left, up to the last, close to the futtock shrouds.
1980, Richard W. Unger,The Ship in the Medieval Economy 600-1600, page34:
That meant it was not possible to useratlines — that is, to make rope ladders out of the shrouds by adding small connecting pieces of ropes.
ratlines on Wikipedia.Wikipedia