The 'International Map of the World (IMW)' was a project which started in 1913 to create a complete map of the world as per internationally agreed and consistent specifications. A map numbering system was also developed to allow identification of maps. Although the mapping project was abandoned by the 80s, the map numbering system continues to be used.
Map numbering is of the form 'NA-12A':
The first letter is either 'N' or 'S' to denote the hemisphere (North/South) with respect to the equator.
A second letter denotes the 4° latitude band with alphabet increasing as latitude moves away from equator. So sheets between 12°N to 8°N would be 'NC' and between 4°S to 8°S would be 'SB'.
The next number denotes the 6° longitude band with numbering starting at 1 from 180°W (i.e. 1 between 180°W and 174°W) going on to 60 (i.e. between 174°E-180°E). So sheet between 72° to 78°E and 12°N to 8°N will be 'NC-43'. Similarly between 132°W to 126°W and 4°S to 8°S would be 'SB-9'. The longitude band span increased to 12° from 60° latitude and to 24° from 76° latitude due to the decrease in physical span of longitude band.
Each 6°×4° rectangle is further subdivided into 24 squares of 1°×1°. Each square is indicated serially by an alphabet increasing first towards east and then towards south, starting with 'A'.
So for sheet between 74° to 75°E and 11°N to 10°N we get 'NC-43I':
For sheet between 128°W to 127°W and 6°S to 7°S we get 'SB-9Q':
The International Map of the World series is limited only upto 1:250,000 scale.