In the United States utility patents have a term of 20 years, and design patents have a term of 15 years (plant patents have a term of 20 years, however, these types of patents are quite rare). The term of a utility patent is taken from the earliest filing date. And, for design patents, the term of 15 years is quite new. Design patents that issued prior to mid May 2015, have a term of 14 years. Furthermore, it is often the case, that a maintenance fee was not paid at one of the required payment periods (3.5, 7.5 or 11.5 years). In such a case, the patent will be expired prior to the full term due to the failure to pay a maintenance fee.
What does expired mean? A patent that has truly expired is in the public domain. When in the public domain, the patent monopoly conferred by the patent is exhausted. And, anyone can make, use, sell and offer to sell the invention that was described and that was covered by the patent. There can be limitations to the foregoing, such as, for example, if a maintenance fee was missed unintentionally or unavoidably. If that is the case, then the patent can be reinstated with the filing of the proper petitions and the like setting forth facts that the USPTO deems satisfy the burden of showing unintentionality or unavoidability.
The greater difficulty comes in when someone finds a product and wants to determine if the patents on that product have expired. In many instances, there may be a patent number on the product. Yet, this does not convey the entire scope of inquiry. For example, more than one patent may cover a particular device, and notice may not be provided to other patents. This is a common occurrence when the patent number is molded into the product (and the patent owner does not want to alter the mode with additional patent numbers). There are also different types of protection that may have been sought. The product may be covered by more than one utility patent, by more than one design patent, or by a combination of design patents and utility patents. In addition, certain features may be protected by trademark or copyright. For this reason, just because one patent has been identified, and that patent has expired, this does not mean that everything is free and clear.
Second, there is the issue that when reviewing an expired patent, it is often the nature of inventors to think of improvements to that which is shown in the expired patent. This process is often sped up when an inventor or innovator or tinkerer starts to embark on the actual process of making the device. These improvements are often highly patentable and entitled to patent protection. Additionally, there can be ornamental type improvements which may be entitled to design patent protection. Thus, while the initial motivation to make and sell a product may be driven by finding an expired patent, this can change quickly to making and selling an improved product which itself may be the subject of a new patent.