An open letter is the opposite of a private letter. It is made available to the public in order to make a larger number of people aware of its content. Such a letter may be addressed to a large group of people — i.e., an open letter to a publication's readers — or it may have a single addressee, such as the President of the United States, whom the letter writer wishes to address in a public forum.
This type of communication may be used to address a single person whose identity the writer does not know, as in the Kids in the Hall sketch in which Bruce McCullough reads an open letter to "the guy who stole my bike wheel." In this case, the open letter is an ideal format because the wider audience gives the writer a chance to meet the intended recipient. Alternatively, it may have no explicit recipients at all, but may simply be used to explain the author's viewpoint or intentions to any who are interested.
An open letter may appear anywhere you would expect to see any public message. Some popular forums include blogs, web forums, magazines, and pamphlets. Many company websites feature open letters explaining corporate philosophy or policies. They may also be read aloud or extemporized on television or the radio, as well as on the stage — or in any public area, for that matter.
Many people find that an open letter can be the perfect way to inform a larger audience about their concerns or to open up dialogue on issues they are interested in. The more people such a letter can reach, the more influence it can have, so consider how many people will have access to the media you choose.
Some famous open letters throughout history include the Pauline Epistles of the New Testament, Emile Zola's J'Accuse!, and Bill Gates' Open Letter to Hobbyists.