F

A set of wooden blocks, with one of them featuring the letter "F". (TV: The Doctor Dances)

As spoken by Sarah Jane Smith, F was the sixth letter of the alphabet, following E. (TV: The Masque of Mandragora)

"F" 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)