In dinghy sailing and some other small sailing boats which do not have their own keel, a removable one is provided for by a centreboard. It is a strong, flat board usually hinged, that can be swung down beneath the boat as the depth of water permits. It prevents the boat being driven sideways by the wind, in effect helping to convert a sideways thrust into forward motion.
The slot which accommodates the centreboard is called a centreboard trunk. When sailing downwind it is usually hauled up as it increases drag. If the swell is great leaving it down can help stop yawing from side to side.
A daggerboard is a lighter version and can be pulled entirely out of the trunk like pulling a dagger from a sheath.
Leeboards are found on some coastal vessels particularly of Dutch origin. They are built onto the sides of the boat where they can be lowered into the water to achieve the same effect as a keel or centreboard.