A knight banneret, sometimes known simply as banneret, was a medieval knight ("a commoner of rank") who led a company of troops during time of war under his own banner (which was square-shaped, in contrast to the tapering standard or the pennon flown by the lower-ranking knights) and was eligible to bear supporters in English heraldry.
The military rank of a knight banneret was higher than a knight bachelor (who fought under another's banner), but lower than an earl or duke; the word derives from the French banneret, from bannire, banner, elliptical for seigneur - or chevalier banneret, Medieval Latin banneretus.
Under English custom the rank of knight banneret could only be conferred by the sovereign on the field of battle. There were some technical exceptions to this; when his standard was on the field of battle he could be regarded as physically present though he was not. His proxy could be regarded as a sufficient substitution for his presence.
As there were no standing armies (except the military orders), military service was rendered ad hoc as an obligation of a vassal, either in person and/or with a contingent raised by one's own means. This social role was crucial: a suzerain, or feudal overlord, was dependent upon his vassals to mobilise on his behalf in case of war. The only alternative was to replace knighthood as the core of military forces with mercenaries, as under a condottiere, but those often proved highly unreliable and expensive, as well as being known for changing sides for greater profit, or simply deserting and looting for themselves.
In feudalism, the rank was given to those nobles who had the right to lead their vassals into battle under their own banner. Ultimately bannerets obtained a place in the feudal hierarchy between barons and knights bachelor, which has given rise to the idea that they are the origin of King James I's order of the baronet. John Selden, indeed, points out that the "old stories" often have baronetti for bannereti, and he points out that in France the title had become hereditary; but Selden is careful to say that "banneret hath no relation to this later title [of baronet]". The title of knight banneret, with the right to display the private banner, came to be granted for distinguished service in the field. No knight banneret, says Selden, of the English custom: