On a yacht wHat is a sheet?

On sailing vessels ropes are called sheets. The rope that is used to control the main sail, for example, is known as the main sheet. Ropes that control the jib sail are known as jib sheets, etc. The expression "three sheets to the wind" to describe someone who is inebriated, comes from the fact that if three of the ropes that control sails were loose and "to the wind" the vessel would be out of control.