Although it’s name may related to a common form for “Sailor”, The Jack Line was considered indispensable to a Deck Seaman’s life. This was a cleated Rope which ran from The Bow to Stern of a Ship (although sometimes it only ran Amidships) – and was used as the primary “safety harness” when Crewmen had to move around the Deck of a Ship in inclement weather. To accomplish this, a Sailor tied a rope harness belt around his waist. To this was attacked a metal or roped loop (loosely). The Sailor (in theory), could pull himself around Deck during bad weather without fear of being tossed overboard.
Some more experienced Seamen often just used their hands minus a harness to traverse a Deck. The use of a Jack Line was a standard component with both Merchant and Warships from the 17th Century onward. Even though with advanced technology, Jack Lines can still be found today on Yachts. Even modern Warships (although no longer powered by sail), have Jack Line cables built into their Main Deck Gunwale Side.