T
From Tardis Wiki, the free Doctor Who reference
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)
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.
