To assist in the comparison of ranks in the armed forces of different countries, NATO rank codes are used. These are established codes for determining the seniority of officer and other ranks in NATO countries for a particular joint task group or command structure, although specific appointments designate a higher level of seniority over other equivalent rank codes in a given situation. Officer ranks go from OF-1 (applying to all subaltern officers below captain) up to OF-10; OF(D) being a special category for trainee officers awaiting a commission. Other Ranks (those considered enlisted men in the United States forces) are classified from OR-1 to OR-9. Warrant Officers in the United States forces are a special case as they are single track career specialists between the enlisted and officer ranks in terms of seniority, and have no NATO rank code. The system should not be confused with the pay grades used in the US military.While countries outside the NATO command structure do not strictly fall into this classification structure and there may be incidences of overlap (particularly in OR ranks between countries), the codes still provide a useful gauge in determining what ranks are broadly equivalent.
These are the highest ranks. Promotion to OF-10 rank is now rare and usually reserved for wartime.