T

As spoken by Sarah Jane Smith, T was the twentieth letter of the alphabet, following S and preceding V. (TV: The Masque of Mandragora, Goodbye, Sarah Jane Smith)

Graffiti reading "TP8" in red was present in London on 25 December 2006. (TV: The Christmas Invasion)

Luke Rattigan told Jo Nakashima that his name was correctly spelt with two Ts. (TV: The Sontaran Stratagem)

Behind the scenes
In the real world, T is immediately followed by U.