-l-, -ll-Much confusion is caused by differing spelling practice in BrE and AmE in verbs of two syllables pronounced with the stress on the second syllable, e.g. enthrall/enthral and fulfil/fulfill. Practice varies even within each variety of English; the following table lists the recommended spellings:BrE / AmEannul / annulappal / appallbefall / befalldistil / distillenrol / enrollenthral / enthrallextol / extolfulfil / fulfillinstall / installinstil / instillThe recommended spellings for nouns in -ment are:BrE / AmEannulment / annulmentenrolment / enrollmententhralment / enthrallmentextolment / extolmentfulfilment / fulfillmentinstalment / installmentinstilment / instillmentFor the spelling of inflected forms of verbs in -l (appal, appalled, appalling, etc.) see doubling of final consonants in inflection.
Modern English usage. 2014.