Larry wants to trip up his oldest son, Dave, who likes puzzles. Larry has created a devious puzzle which he knows his son will never figure out.

On their kitchen table he places a set of paper notes. On each note is a letter. On one of the notes, the letter is missing. He tells Dave to fill in the missing letter.

The notes read:

O, T, T, F, F, [Blank], S, E, N, T.

What letter belongs on the note titled [Blank]?