On occasion, it might be necessary to write a letter to someone asking for information.  I laugh at the thought, but yes there are people who do not use email.

With the ease of email, clarifications about queries can easily be made in a timely manner.  With hand written communication, not so much.  Patience is required when dealing with snail mail.  Not everyone is waiting for your letter and will be able to write back immediately.  Some repositories only have one person who may be answering correspondence, and for example might only do that every second Tuesday.

Please remember that if you are writing to a location where English is not the primary language to keep it simple when asking a specific question, and do not use slang terms.  Also do not ask the repository to give you “anything dealing with name xyz”.

Val Greenwood has a great section on how to write a request for information letter in The Researcher’s Guide to American Genealogy.