The keel is often the first part of a ship's hull to be constructed, and laying the keel, or placing the keel in the cradle where the ship will be built, may mark the start time of its construction. Large, modern ships are now often built in a series of pre-fabricated, complete hull sections rather than being built around a single keel, so the shipbuilding process commences with the cutting of the first sheet of steel.

The most common type of keel is the "flat plate keel", which is fitted in most ocean-going