A letter is a written message conveyed from one person (or group of people) to another through a medium. The term usually excludes written material intended to be read in its original form by large numbers of people, such as newspapers and placards; however even these may include material in the form of an "open letter". Letters can be formal or informal. Besides being a means of communication and a store of information, letter writing has played a role in the reproduction of writing as an art throughout history. Letters have been sent since antiquity and are mentioned in the Iliad. Historians Herodotus and Thucydides mention and use letters in their writings.