The longest English word typeable using only the top row of a letters is “rupturewort”

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The longest English word typeable using only the top row of letters has 11 letters: rupturewort. Similar words with 10 letters include: pepperwort, perpetuity, proprietor, requietory, repertoire, pourriture and typewriter. The word teetertotter (used in North American English) is longer at 12 letters, although it is usually spelled with a hyphen.