Two armoured riders meet on a battlefield with their faces covered by their helmets. Each carries a distinctive painted design on his shield and banner. What is this system of designs called, and what was its practical use?

Explanation

Once helmets covered the face, a personal design on shield and banner was the only quick way to tell friend from enemy and to know who had done what in the field, and such designs became hereditary within families. Reading them as national flags fails because they identified a particular person and lineage rather than a country, and kingdoms had no national flags in the modern sense.