Captain, from the Old Valyrian “caput,” or head, is a military rank identifying the leader of a company. A company is equivalent to the number of troops mustered by a baron or a banneret, so these are the most common occupants for the captain role. If a captain is a baron, his letters patent already give him authority as a commissioned officer of the Army of Valyria. Otherwise, captains are recognized by commissions from the Crown.
In field combat, a company is usually the combination of several retinues, which are the military units led by knights under their individual pennons. For this reason, captains are lords, and usually have a feudal relationship with their retinue leaders as lord to vassal which mirrors their military relationship as captain to lieutenant or cavalier. It is not essential that a retinue leader be sworn to a captain, however, and sometimes knights bachelor bring their retinues under a banneret without also offering fealty to that banneret.
In the Valyrian Navy, the rank of captain is given to any commanding officer of a vessel which has an occupancy that can approximate the minimum size of a field company. Because the class which bears expertise in sailing is not usually the same as that of the feudal overlords, but rather, the merchant classes, naval captains typically may not be lords or even titled individuals, necessarily. Successfully leading a warship into battle is one certain way to gain the recognition necessary to earn a title, however, and many commoner naval captains end up as squires or knights.