Copyright laws apply to original works that are in a fixed medium and are attributed to a specific author or group of authors. This original work may be in the form of a written document, book, song, process, method, idea, etc. The purpose of copyrighting an original work is to control or limit its use by the original authors and to properly credit the original proponents.
However, it is not possible to copyright a phrase given that this is difficult to keep track of its use no matter how unique it is. It is possible to trademark a short phrase, but no copyright laws exist to provide legal protection for a phrase. The US Patents and Trademarks Office offers the possibility to trademark a short phrase, making it easier to track any unauthorized use in a commercial capacity.
A copyright law would only work if the author were diligent enough to control the use of the phrase, which is difficult since a phrase may come to be used in common conversation. A trademark is an alternative that allows authors to easily prove the unauthorized use of any short phrase. An example of a trademarked short phrase is Nike’s tagline, Just Do It.
In order to trademark a short phrase, the principal author must fill out a Trademark/Servicemark Application at the US Patents and Trademarks Office as the principal register. If it is approved, the original authors have the ability to enforce a trademark. If a registered trademark is used without authority, the original authors may bring charges against those who use it without consent.