Everything else in this subgroup is syntax: you build a phrase — ο ένας τον άλλον, μεταξύ τους — and drop it into the sentence. αλληλο- is different in kind. It is word-formation: the mutuality is welded into the word in the dictionary, before any sentence exists. That is why it lives at C1, and why the honest advice at the end of this page is recognise it, and say something else.
The prefix descends from the ancient reciprocal pronoun ἀλλήλων, whose modern remains — genitive αλλήλων, accusative αλλήλους, and the missing nominative — are set out on saying "each other". The pronoun is dead; the prefix is very much alive.
The form: αλληλο- and αλληλ-
The rule as usually stated is simple: αλληλο- before a consonant, αλληλ- before a vowel.
- αλληλο- + βοήθεια → αλληλοβοήθεια
- αλληλο- + γραφία → αλληλογραφία
- αλληλ- + επίδραση → αλληλεπίδραση
- αλληλ- + εξάρτηση → αλληλεξάρτηση
- αλληλ- + ένδετος → αλληλένδετος
What the rule does not tell you is that the elided form belongs almost entirely to the inherited learned vocabulary — words Greek received ready-made from the scholarly register. When a journalist or an academic coins something new today, the prefix stays intact as αλληλο- even in front of a vowel, and the hiatus is marked in writing with a diaeresis wherever the two vowels would otherwise be read as a digraph: αλληλοϋποστήριξη, αλληλοϋποστηρίζονται (without the dots, -ου- would be one vowel). Before other vowels no diaeresis is needed: αλληλοεξόντωση, αλληλοαναίρεση.
On verbs: the productive slot
The living pattern is αλληλο- + a mediopassive verb. The prefix supplies "mutually"; the -ονται / -ούνται ending supplies the fact that the parties are simultaneously doing and undergoing. Journalism produces these by the dozen, and they are transparent on sight to any Greek speaker even when the particular word has never been printed before.
| Formation | Meaning | Where you meet it |
|---|---|---|
| αλληλοκατηγορούνται | they blame each other | politics, news |
| αλληλοϋποστηρίζονται | they support each other | news, reports |
| αλληλοβοηθιούνται | they help each other | news, social writing |
| αλληλοσυμπληρώνονται | they complement each other | academic prose |
| αλληλοαναιρούνται | they cancel each other out | argument, criticism |
| αλληλοσυγκρουόμενες (participle) | mutually conflicting | reporting, law |
Τα δύο κόμματα αλληλοκατηγορούνται για την κρίση και κανείς δεν αναλαμβάνει ευθύνη.
ta dhío kómata alilokatighorúnde ya tin krísi ke kanís dhen analamváni efthíni
The two parties are blaming each other for the crisis and nobody is taking responsibility. (formal / journalistic)
Οι δύο θεωρίες δεν συγκρούονται, αλληλοσυμπληρώνονται.
i dhío theoríes dhen singrúonde, alilosimblirónonde
The two theories don't conflict — they complement each other. (academic)
Οι δύο ισχυρισμοί αλληλοαναιρούνται μέσα στην ίδια παράγραφο.
i dhío ischirizmí aliloanerúnde mésa stin ídhia parághrafo
The two claims cancel each other out within the same paragraph. (academic)
Έφτασαν αλληλοσυγκρουόμενες πληροφορίες από το πεδίο.
éftasan alilosingruómenes pliroforíes apó to pedhío
Conflicting reports came in from the field. (formal / journalistic)
A handful of active verbs exist too, and they are exactly the inherited ones — αλληλεπιδρώ ("interact"), αλληλογραφώ ("correspond"). Note that only the first shows the elided prefix, and only because a vowel follows: age decides whether a word may elide, the next sound decides whether it does. The tendency to hold on to is about voice — inherited formations are often active, coined ones are mediopassive.
Τα δύο συστήματα αλληλεπιδρούν σε πραγματικό χρόνο.
ta dhío sistímata alilepidhrún se praghmatikó chróno
The two systems interact in real time. (academic)
Αλληλογραφούσαμε δέκα χρόνια πριν γνωριστούμε από κοντά.
aliloghrafúsame dhéka chrónia prin ghnoristúme apó kondá
We corresponded for ten years before we met in person. (formal)
Why the prefix earns its keep
There is a real reason a careful writer reaches for it rather than for the bare middle, and it is not just weight. A mediopassive on its own is ambiguous: κατηγορούνται is either "they blame each other" or "they are accused", and formal prose is exactly the register where an από-agent might be lurking a few words later. αλληλοκατηγορούνται can only mean "they blame each other." The prefix closes off the passive reading in a single morpheme — no extra phrase, no risk of misreading.
That is the same job that μεταξύ τους and ο ένας τον άλλον do for the spoken language, done with word-formation instead of syntax. It also explains why the prefix clusters in headlines: it is the shortest unambiguous reciprocal Greek has.
Οι κάτοικοι του χωριού αλληλοβοηθιούνται στη συγκομιδή, όπως πάντα.
i kátiki tu choriú alilovoithiúnde sti singomidhí, ópos pánda
The villagers help each other with the harvest, as they always have. (formal / journalistic)
Τα μέλη της ομάδας αλληλοϋποστηρίζονται και δεν αφήνουν κανέναν πίσω.
ta méli tis omádhas aliloipostirízonde ke dhen afínun kanénan píso
The members of the team support each other and leave nobody behind. (formal)
On nouns: a scale of lexicalisation
The nouns are where the honest work has to be done, because they are not all the same kind of word. Some are living compounds you can decode; others stopped meaning "each other" centuries ago and are now simply vocabulary. Reading αλληλογραφία as "mutual writing" will not mislead you badly, but reading αλληλουχία as anything to do with reciprocity will.
| Word | Means | How much "each other" is left |
|---|---|---|
| αλληλεπίδραση | interaction | all of it — transparent |
| αλληλεξάρτηση | interdependence | all of it — transparent |
| αλληλοκατανόηση | mutual understanding | all of it — transparent |
| αλληλένδετος | interlinked, bound up together | most of it |
| αλληλογραφία | correspondence, letters | a trace — you would not think of it |
| αλληλεγγύη | solidarity | none — a word in its own right |
| αλληλουχία | sequence, coherent order | none — no mutuality at all |
Η αλληλεγγύη των γειτόνων κράτησε την οικογένεια όρθια.
i alilengíi ton yitónon krátise tin ikoyénia órthia
The neighbours' solidarity kept the family on its feet.
Η αλληλεπίδραση των δύο φαρμάκων μπορεί να είναι επικίνδυνη.
i alilepídhrasi ton dhío farmákon borí na íne epikíndhini
The interaction between the two drugs can be dangerous.
Βρήκα την αλληλογραφία του παππού μου σε ένα κουτί κάτω από το κρεβάτι.
vríka tin aliloghrafía tu papú mu se éna kutí káto apó to kreváti
I found my grandfather's letters in a box under the bed.
Note how ordinary the last two feel. Lexicalisation cuts both ways: it is what makes αλληλεγγύη and αλληλογραφία everyday words that any speaker uses without a second thought, while a freshly coined αλληλοκαλύπτονται stays firmly in the newspaper.
The register ladder, honestly
Three ways to say that two people help each other, in three registers, meaning the same thing:
| Form | Register | Risk |
|---|---|---|
| αλληλοβοηθιούνται | (formal) — written | sounds like a report if you say it out loud |
| βοηθάνε ο ένας τον άλλον | neutral — say this | none |
| βοηθιούνται | everyday, compact | shares its form with the passive |
The reflexive counterpart of all this is the prefix αυτο- (αυτοκριτική, αυτοαναιρούμαι), described on reflexive pronoun vs middle voice; both belong to the wider system of learned prefixes on prefixes.
The other formal option: αμοιβαίος
Before you weld anything onto a verb, remember that Greek also has an ordinary adjective for "mutual": αμοιβαίος, αμοιβαία, αμοιβαίο, with the adverb αμοιβαία. It is (formal) but nothing like as heavy as an αλληλο- coinage, and in many contexts it is what a Greek writer would actually pick: αμοιβαία κατανόηση, αμοιβαίο όφελος, με αμοιβαία συναίνεση, αμοιβαίο κεφάλαιο (a mutual fund).
Χώρισαν με αμοιβαία συναίνεση και χωρίς δικηγόρους.
chórisan me amivéa sinénesi ke chorís dhikighórus
They separated by mutual consent and without lawyers. (formal)
Η συμφωνία είναι προς αμοιβαίο όφελος και των δύο πλευρών.
i simfonía íne pros amivéo ófelos ke ton dhío plevrón
The agreement is to the mutual benefit of both sides. (formal)
Common Mistakes
1. Dropping the diaeresis. Greek reads ου as a single vowel, so the dots are not decoration — without them the word says something else.
❌ Η αλληλουποστήριξη των μελών είναι το ζητούμενο.
Incorrect — the ο of αλληλο- and the υ of υποστήριξη must be kept apart: αλληλοϋποστήριξη.
✅ Η αλληλοϋποστήριξη των μελών είναι το ζητούμενο.
i aliloipostíriksi ton melón íne to zitúmeno
Mutual support among the members is the goal. (formal)
2. Translating "mutual friend" with αμοιβαίος. αμοιβαίος describes something that travels in both directions; a person shared by two people is κοινός.
❌ Γνωριστήκαμε μέσω ενός αμοιβαίου φίλου.
Incorrect — a shared person is κοινός: μέσω ενός κοινού φίλου.
✅ Γνωριστήκαμε μέσω ενός κοινού φίλου στη δουλειά.
ghnoristíkame méso enós kinú fílu sti dhuliá
We met through a mutual friend at work.
3. Using αλληλο- as if it were a word meaning "each other". It marks mutuality between two parties; it cannot sit on a verb with one subject acting on one object.
❌ Ο Νίκος αλληλοβοηθάει τον αδερφό του κάθε μέρα.
Incorrect — αλληλο- needs two parties acting on each other, not a subject and an object: Ο Νίκος και ο αδερφός του βοηθάνε ο ένας τον άλλον.
✅ Ο Νίκος και ο αδερφός του βοηθάνε ο ένας τον άλλον κάθε μέρα.
o Níkos ke o adherfós tu voitháne o énas ton álon káthe méra
Nikos and his brother help each other every day.
4. Same meaning, two registers. Both of these are correct; one of them belongs in a newspaper and the other in a conversation.
Οι δύο υπουργοί αλληλοκατηγορήθηκαν στη Βουλή.
i dhío ipuryí alilokatighoríthikan sti Vulí
The two ministers blamed each other in parliament. (formal / journalistic — a written register)
Οι δυο τους κατηγορούν ο ένας τον άλλον από την πρώτη μέρα.
i dhio tus katighorún o énas ton álon apó tin próti méra
The two of them have been blaming each other since day one. (neutral — what you would say)
5. Two nouns from the same prefix, only one of them still transparent. Decoding the second one from its parts will give you the wrong idea entirely.
Η αλληλεξάρτηση των δύο οικονομιών φάνηκε καθαρά στην κρίση.
i alileksártisi ton dhío ikonomión fánike kathará stin krísi
The interdependence of the two economies showed clearly in the crisis. (transparent — 'depending on each other')
Η αλληλουχία των γεγονότων δεν βγάζει νόημα.
i aliluchía ton yeghonóton dhen vgházi nóima
The sequence of events makes no sense. (opaque — nothing reciprocal survives in the word)
Key Takeaways
- αλληλο- is derivation, not syntax. The mutuality is inside the word before the sentence starts — the opposite of ο ένας τον άλλον, which you assemble as you speak.
- The form rule (αλληλο- before a consonant, αλληλ- before a vowel) describes the inherited vocabulary. New coinages keep αλληλο- intact and mark the hiatus with a diaeresis: αλληλοϋποστήριξη.
- The productive slot is αλληλο- + mediopassive verb, coined live in journalism and academic prose. The few active ones (αλληλεπιδρώ, αλληλογραφώ) are inherited.
- The prefix removes the passive reading the bare middle leaves open: κατηγορούνται is ambiguous, αλληλοκατηγορούνται is not. That, not weight, is why formal Greek uses it.
- The nouns sit on a scale: αλληλεπίδραση, αλληλεξάρτηση are fully transparent; αλληλογραφία is just "correspondence"; αλληλεγγύη is "solidarity"; αλληλουχία has no mutuality left at all — and the last three are everyday words you should use.
- αμοιβαίος / αμοιβαία is the lighter formal alternative — but English mutual friend is κοινός φίλος, never αμοιβαίος.
- The C1 target is comprehension, not production: read αλληλο-, say ο ένας τον άλλον.
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Start learning Greek →Related Topics
- Reciprocal: 'each other'B1 — The full inventory of Greek's 'each other' devices: the verb ending, the everyday μεταξύ τους, and the precise ο ένας τον άλλον — how the phrase is built out of two slots (a nominative echo of the subject, then whatever case the verb or preposition wants), why the preposition goes in the middle, the feminine η μία την άλλη and neuter το ένα το άλλο, the genitive ο ένας του άλλου, the sequential ο ένας μετά τον άλλον, and the learned αλλήλων you read but never say.
- Reciprocal Use of the Middle VoiceB1 — How the mediopassive says 'each other' — αγαπιούνται, φιλιούνται, συναντιούνται, αγκαλιάζονται, τσακώνονται, δεν μιλιούνται — and why the requirement is two parties, not a plural subject: Συναντιέμαι με τη Μαρία is just as reciprocal. Covers the singular + με construction, the two disambiguators μεταξύ τους and ο ένας τον άλλον (with the neuter το ένα το άλλο and the case forms), the actives that are reciprocal on their own, and the overlap with the plain passive.
- Reflexive Pronoun vs Middle VoiceB2 — Greek encodes 'myself' twice — with the -ομαι ending and with the noun ο εαυτός μου — and they are not free variants. The middle deletes the object slot, the pronoun fills it, and that single fact predicts every case: why an oblique self, a contrasted self or a modified self forces τον εαυτό μου, why verbs like αγαπάω leave you no choice, why the two can never be stacked, and where the learned prefix αυτο- comes in.