Over the course of the Middle Ages, individual bynames gave way to inherited surnames, so that a man's surname was no longer a literal description except by coincidence. We've chosen to illustrate this discussion with medieval English examples, but the general principles apply to most medieval European languages. The indented paragraphs delve a little more deeply into English naming practices, and don't apply generally. An appendix lists good sources for choosing bynames from other medieval languages. A patronymic byname identifies you as your father's child.
Patronymics are an old and common type of byname in most period European cultures.
Not all cultures used metronymics, though, and patronymics were always more common. There are three main types of patronymic in English. The earliest style in English simply used the parent's name as a byname, e.g. Geoffrey Anketil 1209 was the son of a man named Anketil . When the father's name was used as a byname without modification, we sometime call it an unmarked patronymic . This last type is usually constructed, as in this example, from a pet form of a name that was popular in the Middle Ages, not with full forms of those names or with names carried over in modified form from Old English.