Key
A key is a device to explain the meaning of a map and should include examples of all the symbols used. Preparation of a separate key sheet ensures that visually impaired map readers do not confuse the key with the map. It makes sense to put the symbols on the key sheet in the order in which they are likely to be read on the map. Therefore, it is appropriate to consult one or more visually impaired readers about their about their preferred order of reading a map before designing the key.
In the most successful key layout, braille labels are embossed in a list on a sheet of manilla paper with a Perkins brailler, leaving a left-hand margin of 40.0mm. The brailled sheet is glued to a sheet of card, and the symbols are added within the left margin of the page, with a space of 10.0mm between each symbol and the start of its braille label. Line spacing has to be calculated according to the size of the symbols. Not all visually impaired people read braille, therefore key labels should also be offered in sound form and as a large print underlay. The underlay should be printed in a sans serif font that is easily read by most people and which exactly matches the braille character size and line spacing, such as 26 point Tw Cen MT (Microsoft Word 97) font at a line spacing of multiple 1.02 (see Figure 18).