Dukes are generally blood relatives of a King - non-royal titles generally stop at Earl/Count.The later you go in the Middle ages, the more fully equipped cavalrymen there were, who did not bear the title of knight, so that by the 15th century, they vastly outnumber those bearing the title.
Generally speaking, while William the Conquerer set up some 5000 knights fees in England, and through the high middle ages, several thousand knights were theoretically able to be called upon for service by the crown, by the 15th century, only around 100-150 people bore the title in England.
The title of knight is not inheritable, or at least it was not until the baroque period - as was pointed out, many people who theoretically could be knights, chose not to, because of the expense in maintaining the title, and the obligations incurred, including serving in parliament.
A knight generally commanded a lance, which included the knight, and several soldiers - earlier in Medieval England, this was usually the knight, and a man at arms, and sometimes an infantryman, or a squire or valet, or a crossbowman. Usually 2-3 people at a minimum. By the 14th century, a lance was generally a knight, a valet, and a couple of mounted archers. In other countries, the mix varied - in Italy, mounted crossbowman served in lieu of mounted archers, in Germany, a Gleve generally had a mounted crossbowman, and an infantryman associated, in France and Burgundy, usually a lighter armed cavalryman, and a variety of other types of infantry or mounted infantry, depending on when, where exactly, and the fashion at the time.
A knight of some status could command several lances, more than 10 or 15, anf he might then become a 'banneret', which is really a military title - a knight who was a lawyer rather than a soldier would not be a banneret, since he wasn't leading men in the field. Usually, a banneret was a veryk experienced soldier.
Actual aristocratic titles begin with Baron, the go to count/earl. Later in the Middle Ages, Marquis came to be used, then the royal family King/and dukes. In these cases, the following was determined by land held, and wealth - and also, interest in pursuing military activities.
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