Maybe you have heard this urban legend. An inventor has a brilliant idea, he writes it all down in a letter, places the letter in a sealed envelope addressed to himself, mails it, never opens the letter when he receives it in the mail, and somehow uses the letter to prove that he has patent rights to the invention described within. In reality, this kind of a letter does not secure any rights for the inventor.
A United States patent grants the patent owner the exclusive rights to make, sell, use, or import into the United States the subject matter covered by the claims of the patent. The value of a patent is derived from this ability to exclude others from making use of the material protected by the patent. Generally, if you have invented something and wish to prevent others from copying your idea, you must secure a patent issued by the United States Patent and Trademark Office. Showing a potential infringer a letter proving that you had the idea ten years ago simply is not enough to obtain an injunction preventing the infringer from continuing to use your idea.