The description of arms in this way is called blazonry in heraldic jargon. Again, some knowledge of blazonry is necessary to use the reference books and to interpret the evidence. There are several web sites which provide good general information about both the laws and the language of heraldry. The main contemporary source for medieval heraldry is the rolls of arms. These were practical documents, which often recorded the knights present at a particular battle or tournament.
Heraldic seals provide another contemporary source of information.
It was common for arms to be displayed on seals from about the beginning of the 13th century. Generally, each member of the family would have his or her own seal, and the coat of arms would be encircled by an inscription giving the owner's name. So the impression of a seal attached to a charter has the advantage that, in principle, it is associated with an identifiable person at a particular time. Original seals attached to documents have suffered badly, as a result of both accidental damage and deliberate theft. Fortunately, antiquaries enjoyed drawing seals as well as stealing them, so that there is often a record of what has been lost.