The amman was appointed by the ruler of Brabant and served as the sovereign's representative in Brussels. He held the post at royal discretion. The amman presided over meetings of aldermen and was charged with ensuring municipal compliance with ducal ordinances. He also held police powers, powers of administration, and oversaw all executions within Brussels and the communities encompassing the cuve. In the latter locales he could delegated certain powers to subordinates called vorsters, a kind of local alderman. The exception was Forest, where abbots at the Benedictine abbey held these powers.
The amman first appeared in 1125, when Ascelim is recorded as having been appointed by Duke Godfrey I. The post was created to counterbalance the power of the châtelain, a position that the amman came in time to supplant. The amman lived at the Chambrette de l'Amman and maintained his main office at L'Étoile, both on the Grand' Place. The position was abolished with the close of the ancien régime at the end of the 18th century.
Historical Dictionary of Brussels. Paul F. State.