Is there any common rule of when to append the “о” vowel to “к” preposition in front of words starting with “в”?

Nowadays almost nobody says e.g. “ко врачу,” it sounds quite archaic. I hear a slight semantic difference in intentions gravity, though:

  • Этот отчет должен быть готов ко вторнику.
  • Вечер понедельника удался, и к вторнику я оказался не готов.

“не готов ко вторнику” here was nearly “have not completed some task until Tuesday,” while “не готов к вторнику” states for “did not consider that Tuesday comes so suddenly.”

Are these the alien voices in my head or the meaning really differs?

  • 3
    Really differs. "ко вторнику" means "by the time of Tuesday", "к вторнику" means "(not prepared) for the Tuesday".
    – Anixx
    Commented Jan 13, 2014 at 6:04
  • That’s it: I’ve never heard of other examples of meaningful vowel in such a context. Розенталь too, though (§199.9.) Commented Jan 13, 2014 at 6:22
  • 2
    Being native speaker, I use ко in both cases.
    – Nakilon
    Commented Apr 22, 2015 at 19:40
  • 3
    @Nakilon Being native speaker with a proven linguistic hearing, I wonder why so many native speakers do use improper “ко” in the latter case. Commented Apr 23, 2015 at 4:53

1 Answer 1


This is a preposition that governs the Egressive semantical case, meaning "by the time of" (while the morphological case is Dative).

Egressive is not morphologically independent case in Russian, its forms are always the same as of Dative.

There are other examples:

я готовился к встрече
ко встрече я уже был осведомлен о новостях

In many respects the preposition ко is similar to the preposition во, which is used with Locative. If the word is monosyllabic and has a flying vowel in root, then in Locative case the preposition becomes во, while in Prepositional case it remains в:

во сне я летал
в сне много пользы

лежать во льну 
в льне много пользы

бежать во ржи
в ржи много белка

болячка во рту 

под насыпью во рву

Some other words also require the во for Locative:

Я разочаровался во главе правительства 
(= I became disappointed while being the head of government)

Я разочаровался в главе правительства 
(= I became disappointed with the head of government)
  • 2
    +1, but 'в рте' sounds quite alien, as for me. Can you give any examples of famous writers or any other authoritative sources using it?
    – Yellow Sky
    Commented Jan 13, 2014 at 11:06
  • 1
    @Yellow Sky agreed, removed
    – Anixx
    Commented Jan 13, 2014 at 12:28
  • I agree with examples in the answer but just for the sake of completeness: к вторнику can also be said in the meaning "by the time of". I.e. К вторнику эта задача должна быть решена. Here the speaker could say ко вторнику and the meaning would be the same.
    – farfareast
    Commented Jan 16, 2014 at 1:53
  • @farfareast I would never say so and it sounds weird to me.
    – Anixx
    Commented Jan 16, 2014 at 1:57
  • @Anixx It maybe northern - southern differences in Russian. I guess in the north they tend to omit о more readily.
    – farfareast
    Commented Jan 17, 2014 at 0:04

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