- You may be looking for the components of a writing system.
A letter was a type of written correspondence.
Letters appeared to be a thing of the past by the 30th century, as Stubbs received correspondence on Skybase One from his family on Earth in the form of videotapes, and failed to understand when Jo Grant asked him if he received any letters from them. (TV: The Mutants)
After learning to both read (PROSE: The Dark Path) and write, (PROSE: The Lost) Jamie McCrimmon wrote a letter for Victoria Waterfield, thanking her for her reading and writing lessons, after she departed his and the Second Doctor's company. The Seventh Doctor delivered the letter to an elderly Victoria. (AUDIO: The Story of Extinction)
Reinette Poison wrote the Tenth Doctor a letter in 1764 musing on when he would return to her, hoping he would return before she succumbed to her illness. The Doctor eventually received the letter after Reinette's passing. He read it aboard his his TARDIS. (TV: The Girl in the Fireplace)
Ryan Sinclair's father, Aaron, (TV: Resolution) sent him a letter apologising for failing to attend the funeral of his own mother, Grace O'Brien. He offered Ryan a place to live with him as they were "proper family" unlike Grace's second husband, Graham. (TV: Arachnids in the UK)