Sound laws · Law 5 · Vowels

AU becomes o

Spanish

French

o (spelled o or au)

Italian

Portuguese

ou (said [o] in Brazil)

Romanian

au (kept); ău unstressed

Catalan

What happened

The Latin diphthong AU closed into a single o in Spanish, French, Italian and Catalan; rustic Latin already wavered (CAUDA beside CODA, CLAUDIUS beside CLODIUS). French shows the change came late there: in CAUSAM → chose, the c turned to ch as before an a, so the word still began ca- when French palatalized it. Portuguese stopped halfway at ou, still written though Brazilians mostly say [o]; Romanian kept the diphthong (aur).

Examples

Latin	Italian	Spanish	French	Portuguese	Romanian	Catalan	English relative
AURUM‘gold’	oro	oro	or	ouro	aur	or
LAUDĀRE‘to praise’	lodare	loar	louer	louvar	lăuda	lloar	laud
CAUSAM‘cause, matter → thing’	cosa	cosa	chose	coisa	—	cosa	cause
THĒSAURUM‘treasure’	tesoro	tesoro	trésor	tesouro	—	tresor	treasure
PAUPEREM‘poor’	povero	pobre	pauvre	pobre	—	pobre	poor
AURICULAM (VL ORICLA)‘ear’	orecchio	oreja	oreille	orelha	ureche	orella	auricular

Decoder

Where English, Romanian or a learned word has au, the everyday word in the western sisters usually has o (Portuguese ou): aur → oro, or, ouro; treasure (thesaurus) → tesoro, tesouro.

Exceptions

Learned words keep au: causa, aurora, pausa (es/it/pt/ca), cause, aurore (fr), giving doublets like cosa/causa. Words whose AU had already become o in Vulgar Latin show o or u even in Romanian (ORICLA → ureche, CODA → coadă). French pauvre is pronounced [povʁ]: the au there is a later Latinizing spelling, not a surviving diphthong; Portuguese coisa comes from older cousa.

Watch it happen: AURICULAM

What happened

Examples

Decoder

Exceptions

In your word bank