-rr-

-r-, -rr-

Words of one syllable containing a single vowel a, e, i, o, or u double a final r when a suffix is added (bar, barred, barring; fur, furry; stir, stirred, stirring; but pour, poured, pouring). Words of more than one syllable double the r when the stress is on the final syllable (confer, conferred, conferring; incur, incurred, incurring) but retain a single r when the stress is earlier in the word (enter, entered, entering; offer, offered, offering). Verbs in -fer form adjectives in -ferable (with one r) with the stress on the first syllable (preferable; but transferable has the stress on the first or the second syllable) or in -ferrable (with two rs) with the stress on the second syllable (conferrable), or both (referable, referrable).