In a United States Army platoon, the position of platoon sergeant is normally held by a sergeant first class. The platoon sergeant is the primary assistant and advisor to the platoon leader, a first lieutenant or second lieutenant. When necessary, the platoon sergeant acts as the platoon leader in his or her absence. Unless the platoon leader has extensive prior enlisted experience, the platoon sergeant has more professional military experience due to the difference of time-in-service, since prior military service is not a prerequisite for a second lieutenant's commission. On occasion, when a sergeant first class is not available, a staff sergeant will perform as platoon sergeant.
A sergeant first class is still addressed as "sergeant" except in certain training situations and schools. If a sergeant first class is appointed to the fill the role of first sergeant, he or she is called "first sergeant".
