A patentee who makes or sells patented articles, or a person who does so for or under the patentee is required to mark the articles with the word "Patent" and the number of the patent. The penalty for failure to mark is that the patentee may not recover damages from an infringer unless the infringer was duly notified of the infringement and continued to infringe after the notice.

In other words, there is a big advantage to marking patented products. The advantage is a potential for maximum damages in the event that the patent owner files a lawsuit alleging infringement.

The product should be marked by affixing (e.g., stamping) the word "patent" or the abbreviation "pat.", together with the number of the patent, on the product. When, from the character of the article, this cannot be done, the word "patent" or the abbreviation "pat." together with the patent number should be fixed to a label on the product, or to the packaging for the product.

The marking of an article as patented when it is not in fact patented is against the law and subjects the offender to a penalty. Some persons mark articles sold with the terms "Patent Applied For" or "Patent Pending." These phrases have no legal effect, but only give information that an application for patent has been filed in the Patent and Trademark Office. The protection afforded by a patent does not start until the actual grant of the patent. False use of these phrases or their equivalent is prohibited. Patents may be invalidated due to false marking or marking products as patented when the patent expired.