Sentinel was a unique discipline in Final Fantasy XIV, as it could only be played using off hand equipment - shields. Therefore, it worked as a secondary discipline augmenting a main discipline with the ability to block attacks, among other shield skills. A player could not become a "pure" sentinel, as he must have had a a main hand weapon or tool equipped at all times. Any class that could equip a shield could be paired with the Sentinel Discipline, including Disciples of Magic.