Prior to the 1960's, the US Army classified their tanks as Light, Medium, and Heavy tanks. The last US light tank was the M-41 Walker Bulldog, used by the ARVN (South Vietnamese Army) during the Vietnam War. The last US medium tanks were the M-46, M-47, M-48, and M-60 Patton tanks...the M-46 saw action in the Korean War, the M-48 saw much action in the Vietnam War, and the M-60 saw limited service with the US Marine Corps in Operation Desert Storm in January and Febuary of 1991.
The only true US Army heavy tanks were the M-103's which were retired from the US Army in the early 70's, with the balance of them given to the US Marine Corps; the Marines retired their M-103 heavy tanks in 1974. During the 1960's the US Army adapted the new term "MBT" (Main Battle Tanks) and did away with the old terms of light, medium, and heavy tanks.
During the Vietnam War (three USA battalions were in Vietnam 1/69th, 2/34th, and the 1/77th); US Army Armor Battalions (Tank Battalions) consisted of about 57 tanks. Normally, 3 letter companies (A, B, C), and a HHC (Headquarters and Headquarters Company). HHC had 2 or 3 M-48 Patton tanks (for the Battalion Commander and XO), and the letter companies had three platoons of 5 tanks each, with 2 Pattons in Headquarters Platoon (Company Commander's tank and XO's tank). Two or more Tank Battalions made a Brigade; two or more Brigades made a Division. In Vietnam, the new (built in 1966, and fielded in Vietnam in 1969) M-551 Sheridan light tanks (called Airborne Reconnaissance Assault Vehicles, because the term light tank was not supposed to be used anymore) were assigned to US Armored Cavalry Squadrons; approximately 200 Sheridans were destroyed in the Vietnam War.
AFTER THE VIETNAM WAR; the US Army went to the metric numbering system: 4 MBT's per platoon.