A letter is a written message containing information from one party to another. The role of letters in communication has changed significantly since the nineteenth century. Historically, letters (in paper form) were the only reliable means of communication between two people in different locations.
As communication technology has diversified, posted letters have become less important as a routine form of communication; they however still remain but in a modified form. For example, the development of the telegraph shortened the time taken to send a letter by transferring the letter as an electrical signal (for example in Morse code) between distant points. At the telegraph office closest to the destination of the letter, the signal was transferred back into a hardcopy format and sent as a normal mail to the person's home.
This allowed the normal speed of communication to be drastically shortened for larger and larger distances. This required specialised technicians to encode and decode the letter. The facsimile (fax) machine took this one step further: an entire letter could be completely transferred electrically from the sender's house to the receiver's house by means of the telephone network as an image.