The next letter is 'n'. The pattern misses a letter after a pair of letters, for example: 'a' and 'b' are a letter pair, then 'c' is skipped, then the letter pair 'd' and 'e' comes next. For every other letter pair, the order is reversed so they are not in alphabetical order. The next letter pair is 'm' and 'n', and because the pair 'j' and 'k' is in alphabetical order, this pair must be reversed. Therefore, the next letter is 'n'.
