Ordnance Survey County Series maps were based on the Cassini Projection, a transverse cylindrical projection, where a central meridian of longitude forms the point of origin and the line of zero distortion (where all distances are true to scale). Distances along great circles of latitude that meet this central meridian at right angles are also plotted true to scale, but all (North-South) distances parallel to the central meridian are increasingly too great on the map, the farther they are away from the central meridian. It follows that the Cassini Projection is best suited for areas with a relatively narrow extent in longitude, perhaps no more than three or four counties width in the case of the large-scale maps. Following the recommendations of the Ordnance Survey Davidson Committee in 1938, all large-scale maps were to be recast on a new national Transverse Mercator projection with metric sheet lines, subsequently known as the National Grid. From 1945 the new National Grid kilometre square lines were usually overprinted onto the County Series maps. On the County Series maps, most Scottish counties had their own separate county point of origin, usually the nearest triangulation station to the North-South centre line of the county. Sheets were laid out from North-West to South-East in separate county sequences. It also follows that due to the separate origins and the nature of the Cassini projection, areas at the edges of counties do not fit precisely with their neighbouring county sheets, especially at their East-West extents. From the 1890s, the counties originally surveyed only at the six-inch scale (Edinburghshire, Fife, Haddingtonshire, Kinross-shire, Kirkcudbrightshire, and Wigtonshire) along with Kincardine-shire and the detached portion of Dunbartonshire were revised using new county meridians. In addition, from 1914, pressures to reduce the number of county meridians led to four Scottish county meridians being merged: Dunbartonshire, Linlithgowshire, Roxburghshire, and Stirlingshire. In all cases these counties had new sheet lines for the County Series maps, with their sheet numbers often prefixed by the letter ‘N’. All Ordnance Survey six-inch map sheets carried their relevant county name followed by the specific sheet number. Full-sheets, used for most Highland and Island areas, as well as newly-revised sheets from 1920 to 1924, followed the same specifications of the Ordnance Survey first edition six-inch maps of Scotland. Quarter-sheets, dividing the full-sheet area into North-West, North-East, South-West, and South-East quarters, were the more usual practice. For the second and later edition six-inch maps, there were 1,132 full sheets printed, and 6,332 quarter sheets. The Ordnance Survey first edition six-inch maps had been printed from large, engraved copper plates. In contrast, the 2nd and later edition six-inch maps were printed by photo- or helio-zincography. Zincography was significantly cheaper than copper plate engraving, and corrections were easier to make, but it was criticised for producing less clear maps. Photo- or helio-zincography also allowed the automatic reduction of 25 inch maps to the six-inch scale through photographic methods, allowing further economies in map production. The Ordnance Survey six-inch maps were generally uncoloured, but blue wash was used for water features in the 1890s, and red used for contours from ca. 1912. Source.