World Meteorological Organization (WMO) squares is a system of geocodes that divides a world map with latitude-longitude gridlines into grid cells of 10° latitude by 10° longitude, each with a unique, 4-digit numeric identifier.
On the plate carrée projection, the grid cells appear square; however, if the Mercator projection is used, the grid cells appear 'stretched' vertically nearer the tops and bottoms of the map.
On the actual surface of the Globe, the cells are approximately "square" only adjacent to the Equator, and become progressively narrower and tapered (also with curved northern and southern boundaries) as they approach the poles, and cells adjoining the poles are unique in possessing three faces rather than four.
The second digit (x0xx through x8xx) indicates the number of tens of degrees latitude (north in global quadrants 1 and 7, south in global quadrants 3 and 5) of the 'minimum' square boundary (nearest to the Equator), i.e. a cell extending between 10°N and 20°N (or 10°S and 20°S) has this digit = 1, a cell extending between 20°N and 30°N has this digit = 2, etc.
The third and fourth digits (xx00 through xx17) similarly indicate the number of tens of degrees of longitude of the 'minimum' square boundary, nearest to the Prime Meridian.