Vowel Digraphs: αι, ει, οι, ου, υι | Greek Grammar

Greek looks harder to read than it is, because a handful of two-letter combinations don't spell two sounds — they spell one. These are the vowel digraphs: αι, ει, οι, υι, and ου. Once you know that "και" is a single syllable [ke] and "μου" is a single syllable [mu], written Greek suddenly becomes phonetic and predictable. This page teaches the five digraphs, the one wrinkle they create for spelling, and the little two-dot mark that tells you when a pair is not a digraph after all.

The five digraphs at a glance

Digraph	Sound	IPA	Merges with	Examples
αι	"e" as in bet	[e]	ε	και, παιδί, αίμα, ναι
ει	"ee" as in meet	[i]	η, ι, υ, οι, υι	είναι, εικόνα, εκεί
οι	"ee" as in meet	[i]	η, ι, υ, ει, υι	ποιος, οικογένεια, οι
υι	"ee" as in meet	[i]	η, ι, υ, ει, οι	υιός (rare/formal)
ου	"oo" as in food	[u]	— (unique)	μου, σου, τούρτα, ούτε, που

αι = [e]

The pair αι is simply the sound of ε — a short "e" as in English bet. The two most common words in the whole language start you off: και ("and") is [ke], and ναι ("yes") is [ne].

Εγώ και εσύ, μαζί.

eghó ke esí, mazí

You and me, together.

Το παιδί κοιμάται ήσυχα.

to pedhí kimáte ísicha

The child is sleeping peacefully.

That second sentence is a two-for-one: παιδί has αι = [e] ("pe-dhí"), and κοιμάται hides two digraphs — οι = [i] and αι = [e] → "ki-má-te". Nothing in either word is spelled the way it sounds letter-by-letter, yet both are perfectly regular once you know the digraphs.

ει, οι, υι = [i]

Here is the heart of Greek spelling. Three different digraphs — ει, οι, υι — all say the identical sound: [i], the "ee" of meet. And that same [i] is also spelled by the plain letters η, ι, and υ. So Greek has six ways to write one vowel sound.

Πού είναι το κλειδί;

pu íne to klidhí

Where is the key?

Μου αρέσει πολύ αυτή η εικόνα.

mu arési polí aftí i ikóna

I really like this picture.

Ποιος είναι εκεί έξω;

pios íne ekí ékso

Who's out there?

Η οικογένειά μου είναι μεγάλη.

i ikoyéniá mu íne megháli

My family is big.

Read those aloud and hear the sameness: είναι (ει), κλειδί (ει), ποιος (οι), οικογένεια (οι) — every one of those digraphs is a plain [i]. The rare υι survives almost only in υιός ("son", [iˈos]), a formal doublet of the everyday γιος.

💡

ει, οι, υι, η, ι, υ — six spellings, one sound [i]. This is why Greek children (and adults) hold spelling contests: knowing how a word sounds never tells you how to write it. Which spelling a given word takes is a matter of its history and grammar, not its pronunciation — the untangling is the whole job of the spellings of the /i/ sound page.

ου = [u]

The odd one out — and the easy one. ου is the only way Greek writes the "oo" sound [u] of food, and it never means anything else. No competition, no ambiguity: see ου, say [u].

Δώσε μου λίγο νερό, σε παρακαλώ.

dhóse mu lígho neró, se parakaló

Give me a little water, please.

Έφτιαξα μια τούρτα σοκολάτα για τα γενέθλιά σου.

éftiaksa mia túrta sokoláta ya ta yenéthliá su

I made a chocolate cake for your birthday.

Notice how many tiny high-frequency words are this digraph: μου (my/me), σου (your/you), του (his), που (who/that/where), ού in ούτε (neither). You cannot get through one Greek sentence without ου.

The diaeresis (διαλυτικά): "read these apart"

What if two letters that look like a digraph should actually be pronounced as two separate vowels? Greek marks this with the diaeresis (διαλυτικά) — two dots over the second vowel: ϊ or ϋ. The dots are an instruction: don't merge us, read us separately.

Compare the digraph with the diaeresis:

Εγώ και εσύ.

eghó ke esí

You and me. (και = one syllable, 'ke')

Αυτό το προϊόν είναι φθηνό.

aftó to proión íne fthinó

This product is cheap. (προϊόν = pro-i-ón, three vowels sounded)

Without the dots, "προιον" would read as pro-yon with an οι digraph; the diaeresis on ϊ forces the ο and ι apart into [pro.iˈon].

Ήπιαμε νερό από ένα μικρό καΐκι.

ípiame neró apó éna mikró kaíki

We drank water on a little boat. (καΐκι = ka-í-ki)

In καΐκι the accent and the diaeresis sit together on one letter — ΐ — telling you both "this is the stressed vowel" and "it's separate from the α before it".

When you don't need the dots

There's a neat shortcut built into the accent system: if the first vowel of the pair already carries the stress accent, the pair is automatically read apart — no diaeresis required, because the accent has already done the job.

Ο Μάιος είναι ο πιο ωραίος μήνας.

o máios íne o pio oréos mínas

May is the loveliest month. (Μάιος = Má-i-os, no dots needed)

Ο γάιδαρος κουβαλούσε τα ξύλα.

o gháidharos kuvalúse ta ksíla

The donkey was carrying the wood. (γάιδαρος = ghá-i-dha-ros)

In Μάιος and γάιδαρος the stressed ά sits first, so αι can't be a digraph — the split is guaranteed and Greek leaves the dots off. Save the diaeresis for cases like προϊόν and παϊδάκι ("lamb chop", pa-i-ðá-ki), where the stress falls elsewhere and only the dots can break the pair.

Common Mistakes

1. Reading each letter of a digraph separately. και is one syllable [ke], not "ka-i".

❌ ε-γώ κα-ί ε-σύ

eghó ka-í esí

Incorrect — αι is a single sound [e]; και = 'ke', one syllable.

✅ Εγώ και εσύ.

eghó ke esí

You and me.

2. Giving ει/οι an English 'ay/oy' diphthong. They're a flat [i], not "eye" or "boy".

❌ ποιος

póyos

Incorrect — οι is a flat [i], so ποιος is 'pios', not an English 'oy' as in 'boy'.

✅ Ποιος θέλει καφέ;

pios théli kafé

Who wants coffee?

3. Spelling by ear among the /i/ digraphs. "It sounds like [i]" never tells you whether to write ι, η, υ, ει, or οι — that's a per-word fact.

✅ Το κλειδί είναι στην τσέπη μου.

to klidhí íne stin tsépi mu

The key is in my pocket. (κλειδί takes ει, not ι — you learn it word by word)

4. Ignoring the diaeresis and merging a split pair. The two dots mean "read apart".

✅ Είναι ελληνικό προϊόν.

íne elinikó proión

It's a Greek product. (προϊόν = pro-i-ón, thanks to the diaeresis on ϊ)

Key Takeaways

αι = [e] (και, παιδί); ει, οι, υι = [i] (είναι, ποιος, υιός); ου = [u] (μου, τούρτα).
ει, οι, υι collapse into the same [i] as η, ι, υ — six spellings, one sound — the root of Greek's central spelling difficulty.
ου is unambiguous: always [u], nothing else.
A diaeresis (ϊ/ϋ) says "these two vowels are separate": προϊόν, καΐκι.
If the first vowel of a would-be digraph is stressed, it auto-splits and needs no dots: Μάιος, γάιδαρος.

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