In the United States Army, the rank of First Sergeant (abbreviated 1SG) is an OR8/E8 rank above the rank of Sergeant First Class (OR7), and below the rank of Sergeant Major. It is equal in grade to Master Sergeant, although the two ranks have different responsibilities, with a First Sergeant being senior to a Master Sergeant. The rank is abbreviated as "1SG" in the Army. Master sergeants are laterally promoted to first sergeant upon selection by the senior leadership at Department of the Army, depending on available billets and opportunities. Upon reassignment to a non-first sergeant billet, the soldier reverts to their original rank of master sergeant. First Sergeants are generally the senior non-commissioned officers of company (battery, troop) sized units, and are unofficially but commonly referred to as "first shirt", "top", "top sergeant", "top soldier", "top kick", or "top hat", due to their seniority and their position at the top of the company's enlisted ranks. They are sometimes referred to as "second hat" because the Company Commander may entrust them with important responsibilities, even over one of the company's lieutenants.