Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Armies have military rank systems that are often used by other military services such as air forces or marines. 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 up to OF-10; OF being a special category for trainee officers awaiting a commission. Other Ranks are classified from OR-1 to OR-9.