Romer

A Reference Card or "Romer" is a device for increasing the accuracy when reading a grid reference from a map. Made from transparent plastic, paper or other materials, they are also found on most baseplate compasses. Essentially, it is a specially marked-out ruler which matches the scale of the map in use. The scales are laid out in reverse, such that by lining up the numbers given in the grid reference with the gridlines for the square in question, the corner of the romer lies on the location whose grid reference you wish to read. Some, transparent versions, have a small hole at the origin when this is not at the corner of the Reference Card. this allows access to the map such that the location could be marked with a pencil if using the Reference Card in reverse having been given a grid reference to start with. They are used in many types of land navigation.