In Russian, the preposition “o” when meaning "about" becomes “обо” when we say “обо мне” (=about me) and “обо всей книге” (=about all the book) but why do we then say:

  • “о многих книгах » (=about many books) instead of “обо многих книгах” and,

  • “о встрече” (=about the meeting) instead of “обо встрече »,

as in both cases, we have respectively the exact same cluster of consonants “мн…” and “вс…” as those in the cases of “обо мне” and “обо всей…”.

What are the historical or phonological reasons behind such a different behavior of the preposition “o”/"обо" in its meaning “about / on the subject of”?

If the reason for, say, “обо мне” is the ease of pronunciation, why does it not apply as well to “о многих книгах”?

  • Just as a side note, you can't explain the insertion of -о in prepositions just by looking at the following consonant cluster: во дворе - в двери, во ржи - в ржавчине, во имя - в имени. Jun 5, 2022 at 12:04
  • @SergeySlepov во дворе and во ржи are manifestations of locative case, the same words can be used with preposition в but that would mean prepositional case: играть во дворе vs. в дворе много пользы для детей, прятаться во ржи vs. в ржи много витаминов.
    – Anixx
    Jun 5, 2022 at 13:42

2 Answers 2


In Proto-Slavic, this preposition was a merger of two earlier Proto-Indo-European words: one meaning "around" (> Latin ambi- > "ambiguous, ambivalent", > Greek amphi- > "amphibia, amphitheater"), another one meaning "on" (> Greek epi- > "epilogue, epithet", > Latin ob- "observe, oblige"). It did have the final b.

Due to the law of the rising sonority, that final b became impossible before less sonorous consonants (fricatives and affricates).

There were three ways to deal with it:

  1. Insert a prosthetic vowel after the b
  2. Drop the b
  3. Drop the consonant after the b.

Dropping the consonant after the b would change the root, so it was only used when the consonant in question was either another b or v: обить, обонять, оболочка etc. It was not always used: in e.g. овеять the b was dropped; in обобрать the prosthetic was used.

The other two options (ob > obъ and ob > o) were widely used, quite interchangeably. There were probably some finer phonological reasons for choosing one or another option, but they would come with lots of exceptions.

Shortly after that, another process, the fall of the reduced, was added in the mix. The vowel ъ, being a reduced, had either to fall (disappear) or to morph into the full vowel о.

Reduced vowels fell in so called weak positions: before a syllable with a full vowel or a vowel under stress. In strong positions (either under stress or before a syllable with a weak reduced) they morphed into the full vowels.

This meant that the prefix объ- would change into обо- if it was in a strong position, meaning that the next syllable had a weak reduced. In modern Russian, such roots have a so-called "fleeting vowel" in them, which disappears ("flees") as the word changes its form: оборви / обрывать, обопри / опереть, обдери / ободрать etc.

When used as a preposition, in modern Russian, обо is used before single-syllable words with historical чьт, вьс, мьн: обо всём, обо всех, обо что, обо мне. In other cases, о should be used before consonants. But this is quite an artificial rule, and old texts are full of phrases like обо ржи, обо Льве, обо льготе etc. Note that prefixes and prepositions are very closely related in Slavic languages and until quite recently (early XIX century) grammarians didn't make any distinction between them.

In some cases, after the prosthetic ъ had fallen, the consonant clusters like бб, бв, бп appeared again. Some of them were further simplified (опасть, опека, обежать etc.), some left as is.

Russian has a lot of doublets like обвязать / обязать, обниму / обойму, обволочь / облечь etc., where one of the words is native Russian, and another one is a Church Slavonic borrowing. These doublets show the different evolution of this prefix in different Slavic languages, when the roots were the same, but phonological conditions were different.

The fleeting о is a thing in a lot of Russian prefixes and prepositions: c-/со-, ис-/изо-, над-/надо- etc, for the same reason. In these prepositions, however, the historic ъ was not a prosthetic vowel but rather an integral part of the preposition, so it always stayed in its place right until the fall of the reduced.


While I'm not pretending to be the final authority, there are several factors at work:

  1. "обо" tends to appear before (sonority-non-increasing, so not, say, the one in дне) clusters that contain a fleeting vowel; while etymologically, встреча should've had that (moreover, before с - cf. Ukrainian зустричати, confirming it's въ- rather than в(ъ)з-), there is no synchronic pattern that reveals it (other than the heavy cluster itself), unlike мне - меня (note that this is not the standard fleeting pattern!) and especially всём - весь.
  2. "обо" appears less often when the following word is not directly governed by it: "обо всём" is nearly obligatory, while "обо всех интересующем вопросе" is somewhat strange (as what обо directly governs is вопросе). And, mirroring it, "обо многом", while discouraged by prescriptivist sources, is sometimes found, while "обо многих книгах" (where the relevant object is книгах) is even rarer.
  3. "обо" clearly has a preference for monosyllabic nominals, in addition to preference for pronouns rather than nouns: both "мне" and "всем" are monosyllables (as is accusative что: "обо что" belongs to the same pattern, presumably) while "многом" and "встрече" are not; clitics don't count (обо что-нибудь behaves like обо что). The few corpus examples of "обо" before regular nouns are also of monosyllables: обо псе, обо сне, обо дне (all XIX century or before), so the tendency is old. This may be related to the next point.
  4. Some forms are "unproductive", i.e., they only appear in certain specific list of contexts, often memorized; while "обо мне", "обо всём", "обо всех" and "обо что" are easily memorizable, it is not extended to other contexts. (This may also suggest why "обо всяких" is more acceptable than "обо многих" - it involves sequence of the preposition and the root вс'-, which may just be "stored as a whole").
  5. Etymologically, о and об(о) were distinct, only later joining together; that's why, in particular, accusative-case о(б(о)) nearly always allows for б (об стену) except for the обо cases while prepositional-case о(б(о)) only allows об before vowels (??об стене; before [j], it used to be acceptable but shifted out of it in XX century, cf. Marr's "новое учение об языке", plainly unacceptable in modern Russian - only о языке is possible).

This article may be relevant: https://pandia.ru/text/78/162/45400.php

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