This word is so often misspelled that people memorize it as 'one C, two S'. Which spelling is correct?

"Necessary" is spelled with one c and two s's, a pattern often memorized through the little mnemonic "one c, two s," which helps because the word doesn't sound obviously different when misspelled with the wrong letter counts.

"Neccessary" doubles the c instead of the s, and "necesary" reduces the s to a single letter — both common errors that reverse or drop the correct double-letter pattern the word actually follows.

The word traces back to Latin "necesse" (unavoidable), and its persistent misspelling in English is well-known enough that it regularly appears on lists of the most commonly misspelled English words.