In the Southern zone, the letters run from A through P, while in the Arctic and High Arctic zones, they run from A through H. Each map area in turn is split into sixteen map sheets, numbered from 1 through 16 in an identical boustrophedon pattern to the map areas. Each of these 1:50,000 sheets, in turn, was at one time split into eight map sheets, lettered as lower-case a to h, starting at the southeast corner and ending in the northeast corner; these identified 1:25,000 scale maps that were produced from the 1960s to about 1975, then abandoned, and liquidated in the 1980s (along with the scattered coverage of the 1:500,000 and 1:125,000 scale). Not all topographic paper maps produced by Natural Resources Canada strictly follow this rectangular grid pattern. Some maps also cover land in an area which would otherwise be covered by an adjacent map sheet, simply because the latter area does not contain enough land in Canada to warrant a separate printing.