An assistant called me recently, asking whether the other side on a transaction was correct to keep inserting an extra L every time the word instalment appeared in an agreement.
The traditional/British spelling is with only one L; the Yanks now generally use two – so I told the assistant to stick to her guns if she felt strongly about it.
Similarly, the classic spellings are fulfil and fulfilment – but one increasingly sees a doubling of the second L in both. Fulfill is listed in the Oxford English Dictionary Online only in the usage examples up to about 1600, but is now usual in the USA. Take your pick (but I know which way I’d go).
Are there any rules? Yes, but they’re tricky and inconsistent. The Atlantic is (as ever) the great divide – leaving Canadians adrift somewhere in the middle.
UK: double final L if the preceding letter is A (befall, enthrall, install; but appal), single if it’s another vowel (distil, instil, enrol, annul)
US: inconsistent? Our American spell-check wants appall, instill and enroll, but is OK with the other single-L spellings in the previous line
Derivatives of verbs ending in L
UK: double the L, unless it’s preceded by a long vowel sound (so, travelled but failed)
US: only one L (traveled, traveler)
Derivatives of nouns or adjectives ending in L
UK: when you add –ed, –er, or –y, the L is generally doubled (jewelled, jeweller, gravelly; but unparalleled); before –ish, –ism and –ist, not doubled (devilish, liberalism, naturalist; but panelist or panellist – and the Oxford prefers the double-L form)
US: generally not doubled after –ed and –er (jeweled, jeweler); panelist
UK: never double the L (fulfilment, instalment)
US: double away (fulfillment, installment)
Derivatives of words ending in –ll
Sometimes the second L disappears: almighty, almost, already, altogether, always (but NEVER alright; it’s two words, all right), skilful (Yanks want skillful), wilful (willful in the US of A)
The second L used to disappear (and still could, really) in dullness and fullness
Totally confused now?
Next writing tip: fractured French