Under this [The Modern "Size of Command"] system, starting from the bottom and working up, a Corporal leads a Fireteam consisting of three other Marines. A Sergeant leads a Squad consisting of three Fireteams. As a result, a full squad numbers 13 individuals. Squads usually have numbered designations, e.g. 1st Squad.
A Lieutenant commands a Platoon, which can consist of three or four Squads. In Marine infantry units, Rifle Platoons usually consist of three Rifle Squads of 13 men each, with a Navy Corpsman, the Platoon Commander, and a Platoon Sergeant (a Staff Sergeant who serves as second-in-command. A Weapons Platoon replaces the three squads with a 60 mm mortar section, an assault section, and a medium machine gun section. An infantry Platoon can number from 42 to 55 individuals, depending on the service. Platoons are usually numbered (e.g. 1st Platoon) or named after their primary function (e.g. Service Platoon).
A Captain commands a Company, usually consisting of four Platoons (three Rifle Platoons and one Weapons Platoon). His headquarters can include a Gunnery Sergeant and as many as seven others. So a Company can comprise from roughly 175 to 225 individuals. Equivalent units also commanded by Captains are Batteries and Detachments. In English speaking countries, a Company (or troop in the Cavalry or Armor, and Battery in the Artillery) is usually designated by a letter, e.g. A Company. In non-English speaking countries, they are usually numbered.
A Lieutenant Colonel commands a Battalion or a Squadron, often consisting of four Companies plus the various members of his headquarters. A battalion is around 500–1,500 men and usually consists of between two and six companies.
A Colonel commands a Regiment or Group, often consisting of four Battalions (for an Infantry unit) or five to six Air Groups (for a Wing). Battalions and Regiments are usually numbered, either as a separate Battalion or as part of a Regimental structure, e.g. 1/1 Marines in the Marine Corps or 1-501st Infantry in the US Army.
In these latter, abstractions cease to be helpful and it becomes necessary to turn to an actual unit. The 1st Battalion of the 1st Marine Regiment of the 1st Marine Division of the I Marine Expeditionary Force consists of three infantry companies, one weapons company, and one headquarters and service company. Above that, the 1st Marine Regiment (aka First Marines) consists of four such Battalions and one headquarters company. Marine Air Control Group 18 of 1st Marine Air Wing of the III Marine Expeditionary Force consists of four squadrons, one battery, and one detachment, a mix of different-sized units under a regimental equivalent-sized unit.
The next level has traditionally been a Brigade, commanded by a Brigadier General, and containing two or more Regiments. But this structure is considered obsolete today. At the present time, in the U.S. Army, a Brigade is roughly equal to or a little larger than a Regiment, consisting of three to seven battalions. Strength typically ranges from 1,500 to 3,500 personnel. In the U.S. Marines, Brigades are only formed for certain missions. In size and nature they are larger and more varied collections of Battalions than is common for a Regiment, fitting them for their traditional role as the smallest formation able to operate independently on a battlefield without external logistical tactical support. Brigades are usually numbered, e.g. 2nd Brigade.
The level above Regiment and Brigade is the Division, commanded by a Major General and consisting of from 10,000 to 20,000 persons. The 1st Marine Division, for example, is made up of four Marine Regiments (of the type described above), one Assault Amphibian Battalion, one Reconnaissance Battalion, two Light Armored Reconnaissance Battalions, one Combat Engineer Battalion, one Tank Battalion, and one Headquarters Battalion—totalling more than 19,000 Marines. (Within the Headquarters Battalion are one Headquarters Company, one Service Company, one Military Police Company, one Communications Company, and one Truck Company.) An equivalent elsewhere within the same Marine Expeditionary Force (MEF) might be a MEF Logistics Group (MLG) - which is not a regimental-sized unit (as the word "group" implies), but rather a large support unit consisting of several battalions of support personnel. Divisions are normally numbered, but can be named after a function or personage.
