.S. Marshals also have the common law-based power to enlist any willing civilians as deputies.[citation needed] In the Old West this was known as forming a posse, although under the Posse Comitatus Act, they cannot use military troops in uniform representing their unit or the military service for law enforcement duties. However, if a serviceman/woman is off duty, wearing civilian clothing, and willing to assist a law enforcement officer on his/her own behalf, it is acceptable.[