A

As spoken by the Fourth Doctor, A (Morse code: .- (TV: The Man Who Never Was)) was the first letter of the alphabet, preceding B. (TV: The Masque of Mandragora)

In November 1963, science teacher Ian Chesterton set his Coal Hill School class a problem with the letters A, B and C representing the three dimensions. However, Susan Foreman pointed out that he had not acknowledged two additional dimensions: D, which Ian reasoned was time, and E, which Susan revealed was space. (TV: An Unearthly Child) "A" was among the letters written on a set of wooden blocks in Room 802 of Albion Hospital, within which the Empty Child was questioned by Dr Constantine. (TV: The Doctor Dances)

In 2010, Amy Pond wore a gold necklace in the shape of the letter "A". (TV: The Eleventh Hour et al.)