If the patent holder sells, or licenses another to sell a physical embodiment of the invention, no damages start running until the patent holder has given notice of its patent. One way to do so is to give constructive notice by marking the patent number on the physical device or thing. Another is to send actual notice to the alleged infringer, either by letter or by filing a complaint. The earlier of actual notice or marking of substantially all of the devices at issue will start damages accruing for infringement. There is no notice or marking requirement for patent holders whose patents are not practiced, or for patents covering purely methods.