In days of old prior to machinery, cargo on ships was moved from deep in a ships cargo hold to the deck of the ship  using three wooden spars that were tied together with line. Using line and blocks the spars are raised and then using the rigged blocks cargo is lowered into the ships hold from the deck, or from the cargo hold to the ships deck.

 In competition cadets as part of a six member team will rig a gyn using proper seamanship skills. Cadets are assessed on their knots, whippings, head lashing as well as placement of blocks, strops and the load lifted.

Gyn Diagram

The lifting tackle is used to raise the load from in the cargo hold