One could write their letter on one side, then folds and seals into an envelope. 

MEASURES: 11 1/2” X 8 1/2” at the longest and widest opened. 

CONDITION: As you can see it is folded in 4 places and a little wrinkly looking,

 but there are no tears or stains or soiling, SOME YELLOWING. 

By the way, Twain died the day after the comet returned. 

Thank you for looking and for checking out other items in my ART 2 OMEGA store.