Ranks and insignia of NATO navies' officers
Each officer rank in the navy of a NATO country may be compared with the ranks used by any military service in other NATO countries, under a standardized NATO rank scale. This is useful, for instance, in establishing seniority amongst officers serving alongside each other within multinational command structures.
The grades, prefixed OF- (commissioned officers) and WO- (warrant officers) were established in the document STANAG 2116, formally titled NATO Codes for Grades of Military Personnel.
In many navies, two separate ranks fall within the OF-1 grade. These particular ranks, known by various names in different navies, are commonly given the less formal grades of "OF-1a" (more senior) and "OF-1b" (less senior).