Alphabet

An alphabet was a system of letters. There were twenty-six letters in the English language alphabet, the first fifteen of which were A, B, C, D, E, F, G, H, I, J, K, L, M, N, and O; the last nine letters of the alphabet were R, S, T, V, U, W, X, Y and Z. (TV: The Masque of Mandragora, Midnight, Goodbye, Sarah Jane Smith)

While under mental attack by the Mandragora Helix, the Fourth Doctor and Sarah Jane Smith spoke the alphabet backwards to concentrate. (TV: The Masque of Mandragora)

The Tenth Doctor and the entity possessing Sky Silvestry spoke the first fifteen letters of the alphabet simultaneously. (TV: Midnight)

River Song, while unaware of the Twelfth Doctor's identity, threatened to remove their organs in alphabetical order, to which the Doctor facetiously asked "Which alphabet?" (TV: The Husbands of River Song)