1.4 Diphthongs

Definition of Diphthongs

A diphthong is two vowel sounds combined into a single syllable. Instead of holding one steady vowel sound, your mouth glides from the first vowel into the second. In Latin, these are falling diphthongs, meaning the first vowel is stronger and the second vowel trails off.

A few key traits to remember:

• Diphthongs are always long by nature. You never need a macron over them.
• The stress falls on the first vowel element.
• They take longer to pronounce than a single vowel because of that gliding motion.

Types of Latin Diphthongs

Latin has four main diphthongs: AE, OE, AU, and EU. Each one produces a distinct sound in Classical pronunciation.

AE Diphthong

AE is pronounced like the "ai" in English "aisle." In IPA notation, that's /ai̯/.

• caesar → "kai-sar"
• praemium → "prai-mi-um"
• caelum → "kai-lum"

This is one of the most common Latin diphthongs. In later Latin and the Romance languages, AE collapsed into a simple /e/ sound, which is why you'll also hear it pronounced that way in Ecclesiastical (Church) Latin.

OE Diphthong

OE sounds like the "oi" in English "oil." In IPA: /oi̯/.

• poena → "poi-na"
• foedus → "foi-dus"

OE is relatively rare compared to AE and AU. Like AE, it eventually simplified to an /e/ sound in later Latin.

AU Diphthong

AU sounds like the "ow" in English "how." In IPA: /au̯/.

• aurum → "ow-rum"
• causa → "kow-sa"
• laus → "lows"

AU is the most stable Latin diphthong. It kept its two-vowel sound in both Classical and Ecclesiastical pronunciation, and many Romance languages preserved it (Spanish auto, for example).

EU Diphthong

EU has no perfect English equivalent. You glide from an "eh" sound into an "oo" sound. In IPA: /eu̯/.

• heu → "heh-oo" (said quickly as one syllable)
• neuter → "neh-oo-ter"
• seu → "seh-oo"

EU is the least common of the four diphthongs and appears in only a handful of words.

Pronunciation of Diphthongs

Classical vs. Ecclesiastical Pronunciation

These are the two main systems you'll encounter in Latin courses, and they treat diphthongs quite differently:

Notice that AE and OE lose their diphthong quality entirely in Ecclesiastical Latin. So "Caesar" becomes "Cheh-sar" instead of "Kai-sar." AU and EU stay roughly the same in both systems.

Your course will likely specify which system to use. Most introductory Latin courses teach Classical pronunciation.

Regional Variations

Across the Roman Empire, local languages influenced how people actually spoke Latin. Some regions simplified diphthongs to single vowels earlier than others. These regional differences eventually fed into the distinct Romance languages (French, Spanish, Italian, etc.).

Diphthongs in Latin Orthography

Spelling Conventions

• Diphthongs are written as two vowels side by side: AE, OE, AU, EU.
• In medieval manuscripts, you'll sometimes see ligatures where AE is written as æ and OE as œ. These are the same diphthongs, just written as fused characters.
• Since diphthongs are inherently long, macrons are never placed over them.

Historical Changes

Over time, the pronunciation of AE and OE simplified to a plain /e/ sound, but the spelling often stayed the same. This means you'll sometimes see Latin texts where the spelling preserves a diphthong that was no longer actually pronounced as one. Medieval scribes occasionally adjusted spellings based on what they believed the correct etymology was, which sometimes introduced errors.

Diphthongs vs. Monophthongs

Telling diphthongs apart from monophthongs (single, steady vowel sounds) matters for both pronunciation and meaning.

The core difference: a monophthong holds one vowel quality throughout, while a diphthong glides between two. Diphthongs are also longer in duration.

Here are some pairs that show the contrast:

• caelum (sky) has the AE diphthong → "kai-lum." Compare with cēna (dinner), which has a simple long ē.
• poena (punishment) has the OE diphthong → "poi-na." Compare with bonus (good), which has a plain o.
• aurum (gold) has the AU diphthong → "ow-rum." Compare with cūra (care), which has a simple long ū.

When two vowels appear next to each other but belong to separate syllables, that's not a diphthong. Context and vocabulary knowledge help you tell the difference.

Grammatical Significance

Diphthongs aren't just a pronunciation feature. They show up in grammatical endings and help you identify word forms.

Role in Declensions

The diphthong AE appears frequently in noun endings:

• The first declension genitive singular ends in -ae (e.g., rosae = "of the rose").
• The first declension nominative plural also ends in -ae (e.g., rosae = "the roses").
• This means rosae (with the diphthong) is grammatically distinct from rosa (nominative singular). Hearing and recognizing that diphthong tells you which form you're dealing with.

Impact on Verb Conjugations

Diphthongs also appear in certain verb forms. Some verb stems contain diphthongs that shift during conjugation (e.g., caedō "I cut" but cecidī in the perfect tense). Recognizing these patterns helps you parse unfamiliar verb forms as you read.

Diphthongs in Latin Poetry

Metrical Considerations

Because diphthongs are inherently long, they always count as long syllables in Latin meter. This directly affects how you scan a line of poetry. When you're working through scansion, treat any diphthong as long without needing to check further.

Elision and Diphthongs

When a word ending in a diphthong is followed by a word beginning with a vowel, elision can occur. The diphthong gets absorbed into the following vowel for metrical purposes, reducing the syllable count of the line. Some poets deliberately avoid eliding certain diphthongs for stylistic effect.

Common Latin Words with Diphthongs

Building familiarity with these words will help you spot diphthongs quickly as you read.

Everyday Vocabulary

• audi (listen) — AU diphthong
• causa (cause, reason) — AU diphthong
• poena (punishment, penalty) — OE diphthong
• seu (or if) — EU diphthong
• praemium (reward) — AE diphthong

Literary Examples

• Aenēās — the hero of Virgil's Aeneid, with AE at the start
• laus (praise) — common in both poetry and prose, AU diphthong
• foedus (treaty) — an important political and legal term, OE diphthong
• heu (alas) — an exclamation found throughout Latin poetry, EU diphthong

Evolution of Diphthongs

Changes from Old Latin

Old Latin had additional diphthongs that disappeared before the Classical period. For example, the Old Latin diphthong oi shifted to a long ū in Classical Latin. Meanwhile, some diphthongs like EU became increasingly rare, surviving in only a few words. Certain archaic diphthongs hung on in legal language and fixed expressions even after they'd dropped out of everyday speech.

Influence on Romance Languages

Tracking what happened to Latin diphthongs helps explain vocabulary across modern Romance languages:

• AE typically became /e/: caelum → French ciel, Spanish cielo
• AU was sometimes preserved (aurum → Spanish oro) and sometimes shifted (French or)
• OE generally simplified: poena → Spanish pena, French peine

Diphthongs in Latin Loanwords

Greek-Origin Diphthongs

Many Latin diphthongs entered the language through Greek borrowings:

• AE often represents Greek αι (paedagōgus from παιδαγωγός)
• OE sometimes represents Greek οι (oeconomia from οἰκονομία)
• EU frequently represents Greek ευ (eunūchus from εὐνοῦχος)

Diphthongs in Scientific Terminology

Latin diphthongs survive in many English scientific and medical terms. Recognizing them helps with both spelling and comprehension:

• aetiology (study of disease causation) retains AE
• auscultation (listening to internal body sounds with a stethoscope) preserves AU
• oesophagus (the food pipe) maintains OE in British English spelling

American English often simplifies these: aetiology becomes etiology, and oesophagus becomes esophagus. The diphthong disappears from the spelling, but knowing the Latin origin explains why the British spellings look the way they do.