sep

Sep and ses differ by one letter and one place, and mishearing them is the single commonest numerical error in spoken Esperanto. The p and the s are clear enough on the page but not across a noisy congress hall, so speakers often say the number twice or hold up fingers. Everything else is regular: sepa for seventh, sepdek for seventy, sepono for a seventh, sepope for in sevens. Duration takes the accusative without a preposition, hence sep horojn rather than dum sep horoj, though both are correct.