1stSgt

First Sergeant (1SG) is typically a senior non-commissioned officer rank, used in many countries. In NATO armed forces the rank is on OR8 level.

In the United States, a First Sergeant generally serves as the senior enlisted advisor (SEA) of a unit, such as a company, battery, troop, or a USAF squadron or higher level unit. (USA and USMC squadrons and battalions, as well as all higher-level units, have a Command Sergeant Major [USA] or Sergeant Major [USMC] as the SEA.) While the specifics of the title may differ between the United States Army, Marine Corps and Air Force, all First Sergeants can be identified by the presence of a lozenge-shaped (colloquially "diamond") figure on their rank insignia.

Historically, the rank of “First Sergeant” has existed in the American Army since 1781, when a fifth sergeant was added to the table of organization for Continental Army infantry regiments. Previously, under the tables of organization approved by the Continental Congress in 1776 and 1779, there were four and three sergeants, respectively, authorized in each company. The sergeants were numbered in order of seniority and the “First Sergeant” was simply the senior sergeant in the company, but not a separate rank. (Wright, 2006) In 1833, First Sergeant, along with orderly sergeant, became separate pay grades, ranking below Sergeants Major and quartermaster sergeants but above sergeants. While the sergeant major and quartermaster sergeant already had distinctive staff NCO rank insignia, it was not until 1847 that the first sergeant received the lozenge, or diamond, with the three chevrons of a sergeant as his insignia of rank. (The separate rank of orderly sergeant was combined with first sergeant in 1851.) (Perrenot, 2009)

In the United States Army, the rank of first sergeant (abbreviated 1SG) is an E-8 paygrade rank above the rank of sergeant first class (SFC), and below the rank of sergeant major (SGM) or command sergeant major (CSM). It is equal in grade to master sergeant (MSG), although the two ranks have different responsibilities. Both ranks are identical with three chevrons up (standard sergeant insignia) and three curved stripes underneath "down" known as "three up and three down", though the first sergeant has the lozenge "diamond" in the middle. A first sergeant is generally senior to a master sergeant in leadership matters, though a master sergeant may have more general military authority such as when in charge of a military police (MP) section. The rank is abbreviated as "1SG" in the Army.

...
Wikipedia