In this article I would like to explain the difference between logos and coats of arms generally called crests.
After years of experience in heraldry and logo design, I found that these terms confuse not just clients who want me to make them their visual identity, but graphic and logo designers also. Usually when clients want a "coat of arms design", by their own words, they mean in fact a logo inspired by heraldry. I've also noticed that most of the people using the term "crest" when they mean on logo or coat of arms also, even if the "crest" is just a part of it.
My experience in design started in fact with heraldic design, which implies design of coats of arms. Coats of arms design is subjected to strict heraldic rules. Heraldic rules determine the colors, the division of the shield, the position of heraldic elements in the shield, but also (if coat of arms isn't shown just as a shield) the other elements, such as mantle and its colors, type of the crown, colors of the wreath and similar.
The crest itself is composed of the elements (things) above the helmet! So, the crest is not an complete coat of arms, specially not some logo composition which doesn't even comply to heraldic rules.
Let's look at one example of classic coat of arms composition.
Each of the elements of professional/traditional heraldic coat of arms should be a subject of the rules of blazoning. Blazon is textual description of the shield in heraldic terms, which sounds like an encrypted message, but handy heraldic designer should be able to design a coat of arms based on that textual description, once more: called blazon.
In heraldic sense, coat of arms must be shown in the shape of the shield. No shield, no coat of arms! If the shield line is missing, than this isn't coat of arms, we can rather call it an emblem. The other important thing are colors. Coat of arms should be colored. But not in any color. Heraldic colors are several and strictly prescribed.
Let's now see a few examples of logos I created, inspired by classical heraldry. All these logos don't have shield line around them and this is a first request when design of coat of arms is in question. We can rather call them logos or emblems.
In further example there is also one of my logos, even if logo is shown with the shield line, design could not be called a coat of arms because position of the elements around the shield are just of decorative nature and heraldry doesn't recognize such composition of the elements as correct. Again, in heraldry, elements should be composed strictly by heraldic rules.
In fact, most of the designs inspired by heraldry and produced by graphic designers, which can be find named as "crests" and "coat of arms" are made like this - in heraldic sense they can not be called with these names because they don't satisfied the rules.
Of course, I am not saying that all of those logo designs inspired by heraldry are badly done from aesthetic and visual point of view, I am just pointing the difference between the terms and their meaning.
Logos, unlike the coats of arms design, are not the subject of any rules. Designer is free to create whatever he has on his mind.
Ergo: the main difference, as you can see, is in descriptions used to describe them and the problem lies in "not at all" or pure usage of heraldic terminology.
One last confession to make: I myself started to use those confusing expressions, because in presenting my designs on the net I am forced by browsers and search engines to use them; otherwise I will have no visitors at all! :)