What letter comes next in the sequence "O, T, T, F, F"?

The letter that comes next in the sequence O, T, T, F, F, is S. Each letter represents the first letter of each number in the sequence of natural numbers. S stands for "Six."

The sequence "O, T, T, F, F" and similar phrases, such as "O, T, T, F, F, S, S," are mentioned on a variety of brain teaser and trivia sites, such as Chaos Trivia, Braingle and others. The phrase contains the first letter of each natural number: "One, Two, Three, Four, Five." Therefore, the next several terms in the numeric sequence are: "Six, Seven, Eight, Nine, Ten." Based on the nature of the riddle, this is written out as: "O, T, T, F, F, S, S ,E, N, T" and so on.