A troop is a military sub-subunit, originally a small formation of cavalry, subordinate to a squadron. In many armies a troop is the equivalent element to the infantry section or platoon. Exceptions are the Royal Horse Artillery and the US Cavalry, where troop are subunits and refers to an infantry company or artillery battery. A cavalry soldier of private rank is called a trooper in many Commonwealth armies (abbreviated Tpr, not to be confused with trouper).