My cousin wrote this and posted it on another popular genealogy website. I asked if I could re-post it here and luckily the answer was yes. Here goes (please don’t shoot the messenger, LOL):

Firstly, it’s important to know that there really is no such thing as a “family coat of arms”. Arms are assigned to individuals, not families, although the arms assigned to someone holding a hereditary title can be passed on. While there is what can be called a family crest in use in conjunction with arms, that can be carried through from one generation to another, the correct use of the arms themselves has strict limitations. The precise design of the arms and the “permission” for the individual to use the ‘device’ (also a reference to the arms) is very closely controlled by the College of Arms, which is not a college in the academic sense, but an authority in England, Wales, Northern Ireland, and other Commonwealth countries, founded way back in 1484. A similar institution exists in Scotland called the Court of the Lord Lyon. Prior to the organization of these authorities, the use of arms developed in Europe amongst the ruling classes from the use of banners, surcoats, and shield markings in battle and tournaments, as a means of quick visual identification. While there are no fixed criteria nowadays for the applicants to these organizations for arms, there are a few sure bets. As a general rule, all British “titled” (nobility/aristocracy/knighthoods) persons will have been granted arms. (These arms are often the basis for the generic arms designs crafted by companies that sell people a set of arms calling it a family crest or a family coat of arms.)

In the British system a hereditary title generally passes from father to eldest son, unless there is no male heir, in which case it will often “become extinct”. In some cases, especially for nobility, it might pass to a younger brother of the title holder, or a daughter, nephew, or cousin. Frequently a person that inherits a hereditary title (and therefore the use of arms) will seek to modify the arms by adding something significant to the earlier device, or to change the design in some way, like maybe a reference to the arms of a spouse (if that spouse is entitled to arms) or some way to indicate a different generation than the original holder.

This is a very simplified explanation; the modern rules concerning the use of arms is much more complex. For more information, it’s easy enough to find the history of heraldry online using any search engine, and it’s fun to read the terms used in the study and art of heraldry; it’s almost a language of its own. There are also plenty of books on the subject. The College of Arms and Court of the Lord Lyon both have their own excellent websites. Now, that brings me to the modern-day common habit of digging up (or paying for!) a generic coat of arms for everyone in the family tree. As you can tell, that is not an officially sanctioned use of arms, and since an individual entitled to arms has probably made his/hers different and unique in some way, the arms are not correct, anyway. On the other hand, as I understand it, if you are the eldest male and are descended from an unbroken line of descent from the eldest male in each generation of a rightful holder of a coat of arms, you are authorized to carry them.

All of that said, if heraldic accuracy is not a huge priority, and if adding generic coat of arms makes your tree more fun, there’s no real harm in it; as long as you realize that unless it is the right one for the right individual, it’s only a colorful way to decorate your tree. And let’s face it, the worst that could happen is a mildly embarrassing moment or two in some circles. What IS correct and challenging is to find a depiction of the actual arms assigned to individuals in our trees (almost always those that were born in Europe, Britain, or the Commonwealth nations). Hope this clarifies - good luck to all in their research.