different between jackstay vs backstay

English

Jack +? stay

jackstay (plural jackstays)

(nautical) A stay (rope, bar or batten), running along a ship's yard, to which is attached the head of a square sail.
(nautical) A cable between two ships or from a ship to a fixed point which can be used to support a load during transfer of personnel or materiel along the cable.
(nautical) A line (rope, webbing or cable), attached to a boat at the ends, to which a safety harness can be clipped to restrain falling in rough conditions and to prevent falling overboard.
(underwater diving) A line fixed at both ends, which may be used to guide a load or a diver along the route of the line. Uses include guidance to and from the underwater work site, and as a means of controlling an underwater search.

back +? stay

backstay (plural backstays)

(nautical) A part of the rigging of a sailing ship extending from masthead the top of the mast to the back of the ship; they support the strain on all upper masts and provide additional support to the shrouds when the wind is abaft the beam.
A strengthening or supporting piece that is built into the back of something.

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