There was great variation in the spelling of
Middle English, and of
Early Modern English (such as
Shakespeare), but spelling was pretty much fixed in its modern form before 1700: no more flourishes such as
knyghte. Changes since then have been minor. The first
dictionary influential enough to fix
spellings was that of
Dr Johnson in the mid 18th century: he favoured
despatch, for example.
Noah Webster in the early 19th century was a
spelling reformer: he wanted wide-sweeping changes to regularize and simplify
orthography, and he also wanted to create a distinctly American form of language. Where there were two variant spellings in circulation he chose the simpler, but he also invented many. Most of his reforms fell by the wayside but those that remain constitute the bulk of American spelling. In general they are not used outside the USA and to a lesser extent
Canada, though you do sometimes see the
-or words in informal writing in Australia. In the late 19th century the
Oxford English Dictionary was created, and recorded existing spellings, principally British with less emphasis on American ones. No-one at Oxford
decided any spellings: they had crystallized by about 1800 from the same variations Webster worked on.
The following is not a complete list.
common groups
AmE (
American English) has -or in
color,
honor,
valor,
ardor,
neighbor, etc. where the rest of the world has -our in
colour,
honour,
valour,
ardour,
neighbour. But
RotW (
rest of the world, often loosely called
British English) also has -or in some, like
horror,
terror, and agent nouns (
governor,
terminator etc.).
AmE has -er in theater, center, meter, fiber, maneuver, etc. where RotW has -re in theatre, centre, metre, fibre, manoeuvre. But note that RotW uses meter for a calibrated measuring device, and though the AmE spelling saber exists, write-ups here indicate that sabre is in wide use, possibly because of the French origin of fencing terms.
AmE tends to reduce -ae- and -oe- to -e- more than RotW does, but this varies considerably anyway. Initial ae- or oe- is unlikely to be reduced to e- except in AmE, so these are most distinctive: edema, estrogen, for RotW oedema, oestrogen. Spellings such as paleontology, archeology, medieval, are more likely in AmE than RotW but both can occur anywhere. Note that maneuver mentioned above contains this change also.
AmE -g for -gue when the -ue is silent: catalog, analog, decalog, for RotW catalogue, analogue, decalogue. But I don't think I've ever seen pedagog, demagog for pedagogue, demagogue , though the shorter forms exist in Webster 1913.
AmE -m for -mme: television program v. programme, but RotW now also uses -m in telegram, kilogram, and computer program.
AmE -in for -ine in some chemical names, such as adrenalin, glycerin, though these also occur in RotW beside adrenaline, glycerine. But AmE uses -ine in element names chlorine, iodine etc.
AmE -ll for -l in compound words formed from a monosyllable with -ll: such as enroll, instill, distill, fulfill for RotW enrol, instil, distil, fulfil (from monosyllables roll, still, fill). But RotW also uses -ll after -a-, as in install. In compounds: AmE fulfillment, enrollment, RotW fulfilment, enrolment, but also instalment. This is pretty confusing for good spellers anyway, and you're likely to get uncertainty with these.
AmE -ize is often called an Americanism but is in fact the standard form used in British books, though most people and newspapers write -ise. However, AmE only for -yze in analyze, paralyze v. RotW analyse, paralyse.
AmE -nse for -nce in offense, defense for RotW offence, defence. The word license is in theory a verb in RotW, with noun licence, and similary with practise/practice, but this is another one even good spellers are likely to be confused about.
inflectional forms
Final -l and -p after an unstressed vowel remain single when inflectional endings are added, e.g.
worshiped,
worshiping,
traveled,
traveler,
traveling, as against RotW
worshipped,
worshipping,
travelled,
traveller,
travelling. RotW usage with -t and -s is variable: you see
riveting,
rivetting,
biased,
biassed. Similarly
marvelous v.
marvellous, and see fulfil(l)ment etc. above.
AmE uses regular aging, RotW retains the -e- in ageing.
This is not purely a spelling thing, but where two forms of the past tense verb exist in a pair like dreamt/dreamed, AmE is more likely to use the -ed form.
isolated words
airplane v.
aeroplane, and two syllables v. three
aluminum v.
aluminium with different pronunciations (-i- is
official)
ass v.
arse with different vowels (and only meaning bottom)
ax v.
axe
behoove v.
behove (and each pronounced as written)
carburetor v.
carburettor
check v.
cheque (monetary instrument)
cozy v.
cosy
draft v.
draught (in some senses, hard to define)
curb v.
kerb (edge of road)
fuze v.
fuse (explosive, electrical, but -s- also common in AmE)
gray v.
grey (but gray also in Scotland)
jewelry v.
jewellery
mold v.
mould
neuron v.
neurone (but -on is also common in RotW)
omelet v.
omelette
plow v.
plough
pajamas v.
pyjamas
smolder v.
smoulder
sulfur v.
sulphur (-f- is official)
tire v.
tyre (wheel covering)
tranquility v.
tranquillity
woolen v.
woollen