Origin of the Rank and Grades of General
Following tradition tracing back to the Roman title of imperator, a European monarch was always the general of his country’s army. His principal military assistant, in peace and war, was usually called a constable, a member of the nobility renowned for military prowess. Other outstanding noble warriors, particularly in France, frequently carried the honorific title of marshal. When a ruler was present in the field, he automatically exercised command as general. His second in command, who might or might not be the constable or one of the marshals, exercised his military functions as the lieutenant general. In the absence of the monarch, the lieutenant general commanded in the king’s name.
Under the operational command of the monarch or his lieutenant general was a senior administrative officer known as the sergeant major general. An experienced soldier, not necessarily a nobleman, the sergeant major general was in effect the chief of staff. He was responsible for supply, for organization, and for forming up the heterogeneous units of a 16th-century [C.E.] army for battle—a long, complicated process, with much shouting and confusion, considerably helped if the sergeant major general had a stentorian voice. In his administrative functions he was assisted in the subordinate units—national and mercenary—by administrative officers known as sergeants major and sergeants.
There was no permanent military hierarchy or chain of command below king and constable. Lieutenant generals and sergeant major generals were appointed for a campaign only.
— The Encyclopedia of Military History, p. 459
See also Origins of European Army Ranks.