Consider the task of indexing a street map from scratch before computers were readily available. A dye line copy of the map was made to cross out each name as it was written on an index card with its postcode and alpha-numeric grid reference. These cards (3 examples can be seen here) would then need to be sorted alphabetically and typed into a photo typesetter. But, decisions about font, point size, column width and leading (the gaps between the lines) all need to be made to produce the final gazetteer. Guides were used to calculate and measure these variables with scales for points and picas, inches and millimetres. The two most commonly used units of measurement in typesetting, a point is equal to 1/72 inch and the pica 1/6 inch or 12 points.