Preventer
Roger: Generally, a preventer is a short piece of line that you attach to the boom (the bails are a good attachment point) and to some secure spot on the deck (like a cleat or something). Stanchions are not good attachment points and don't use bungee cords. You want to use a preventer primarily when sailing down wind. The idea is to "prevent" or secure the boom from accidently flying across the boat to the other side. With a preventer attached to the boom, if the boat driver makes a mistake or the wind gets fluky and gets behind the sail, the boom can't go anywhere. It is important to always attach the preventer when the boom goes over the side of the boat. It is also important to remember to undo it when you change your point of sail. Every one forgets to untie the preventer every once and a while.
