It seemed strange at first that, not only is the height of the major "squares" inconsistent between rows, it actually increases as one moves North! It then hit me that those horizontal lines are not randomly splitting the map into rectangles, but are in fact latitude lines, apparently marking every 5˚ of latitude. Similarly, the vertical edges seem to mark every 5˚ of longitude. That, in turn, means each minor square marks a 1˚x1˚ geographical area, so one can simply select their choice of location in Google Maps, check the coordinates and figure out the matching major and minor squares.
For instance, the lower left corner of major square O3 has the coordinates 35˚N 10˚W, so Tangier, bordering the Strait of Gibraltar and having the coordinates 35.77˚N 5.82˚W will be in major square O3, minor square 25 (less than a degree north and just over 4 degrees east from the lower left corner of the major square).
As I type this, I realise many of those following this thread are thinking "Well, duh!" because this was obvious to them, but maybe some will find it an interesting bit of info.
