-or - -our - -orous

British and American English differ in the spelling of the noun-ending

• in British English, it is almost always written -our, as in 'rigour', 'glamour', 'odour' and 'valour'
• in American English, it is almost always written -or, as in 'rigor', 'glamor', 'odor' and 'valor'.

It may be of interest to the etymologically curious of AWE's readers that such words as 'emperor', 'governor', 'horror' and 'terror' were written in English with '-our' until quite late: emperour (until about 1700), governour (Sir Thomas Elyot (c. 1490–1546) published The Boke Named the Governour in 1531), horrour and terrour (both till the eighteenth century).

• In both versions of English, adjectives formed from the nouns ending in -our (or -or) are spelled with the termination -orous. (OED records six words ending in '-ourous', of which three are obsolete and three are zoological.)

Some examples follow. (Note that the ending '-orous' may also belong to words whose fuller suffix is -ivorous ('eating', as in 'omnivorous' ('devouring anything'), 'insectivorous' ('eating insects') and 'carnivorous' ('flesh-eating')), -chlorous ('containing chlorine') or -phorous ('carrying', rarely used outside biology and chemistry.)

! Noun British spelling	Noun American spelling	Adjective	Notes
amour	amour	amorous	amour is not as other nouns in this list: last syllable pronounced '-OOR'
clamour	clamor	clamorous
clangour	clangor	clangorous
decorum	decorum	decorous	Although not a noun in -o[u]r, the noun decorum is now naturalized English Also the opposite, indecorous
dolour	dolor	dolorous
glamour	glamor	glamorous
humour	humor	humorous	Also humorist
odour	odor	odorous	Also malodorous, 'evil-smelling'
rancour	rancor	rancorous
rigour	rigor	rigorous
sonor (Latin)		sonorous
stertor	stertor	stertorous	The rare noun stertor means ' a snore'
timor (Latin)		timorous
valour	valor	valorous
vapour	vapor	vaporous
vigour	vigor	vigorous

Languor (in both British and American English), which gives the adjective languorous, should not be confused with this pattern. The '-o-' and the '-u-' are transposed.