"Both" looks like an innocent little word, but in Russian it is a numeral with two faces: о́ба for masculine and neuter nouns, and о́бе for feminine nouns. It behaves grammatically like два/две — same gender split, same government in the nominative/accusative — and like all numerals it declines in the oblique cases. The wrinkle that trips people up is that the gender distinction does not disappear when it declines, the way it does for два (which collapses to двух for both genders). Instead о́ба and о́бе keep separate oblique stems all the way through: masculine/neuter обо-, feminine обе-. Get the vowel wrong — say *в обе́их слу́чаях for masculine слу́чаи — and a native speaker will hear it immediately.
Two forms by gender, like два/две
In the nominative and accusative, choose the form by the noun's gender, and the noun goes into the genitive singular — exactly as it does after два/две (this is the government rule at work).
| Gender of noun | Form | Example (nom./acc.) |
|---|---|---|
| Masculine | о́ба | о́ба бра́та |
| Neuter | о́ба | о́ба окна́ |
| Feminine | о́бе | о́бе сестры́ |
О́ба бра́та у́чатся в одно́м университе́те.
Both brothers study at the same university. (masculine → о́ба + genitive singular бра́та)
О́бе сестры́ верну́лись домо́й по́здно.
Both sisters came home late. (feminine → о́бе + genitive singular сестры́)
В ко́мнате бы́ло темно́: о́ба окна́ занаве́шены.
It was dark in the room: both windows were curtained. (neuter окно́ → о́ба окна́)
The declension — gender split survives
When о́ба/о́бе move into an oblique case, they take plural adjective-like endings, and the gender distinction lives on as a vowel contrast: masculine/neuter built on обо-, feminine on обе-. Memorise the two columns side by side, because the only difference is that one vowel.
| Case | Masc./Neut. | Fem. |
|---|---|---|
| Nom. | о́ба | о́бе |
| Gen. | обо́их | обе́их |
| Dat. | обо́им | обе́им |
| Acc. | = nom./gen. | = nom./gen. |
| Instr. | обо́ими | обе́ими |
| Prep. | (об) обо́их | (об) обе́их |
Once it declines, о́ба/о́бе agree with the noun in case rather than governing it — the same shift you see with every numeral once it leaves the nominative (see declining the numerals).
В обо́их слу́чаях реше́ние бы́ло одина́ковым.
In both cases the decision was the same. (masculine слу́чай → prepositional обо́их + слу́чаях agree)
Она́ держа́ла ча́шку обе́ими рука́ми.
She was holding the cup with both hands. (feminine рука́ → instrumental обе́ими + рука́ми agree)
Я благода́рен обо́им роди́телям.
I'm grateful to both parents. (роди́тели treated as masc./mixed → dative обо́им + роди́телям)
The distinguishing insight
О́ба/о́бе is essentially "the two", a numeral that bundles a known pair into one unit while keeping track of its gender. That is why it patterns on два/две for government (genitive singular in the nominative) yet declines with plural endings (обо́их, обе́ими) — it is counting two specific, already-identified things, so it leans plural in the oblique cases. The feature with no English parallel is the persistent gender split in declension: English "both" is one frozen word, but Russian forces you to track gender not only when you pick о́ба vs о́бе but through every case form, обо́их vs обе́их. The mismatch English speakers must guard against is exactly this — "both" feels invariable to us, so the instinct is to leave it unchanged, while Russian demands a gendered, case-marked form every time.
Common Mistakes
❌ О́ба сестры́ верну́лись домо́й по́здно.
Incorrect — сестра́ is feminine, so 'both' must be о́бе, not о́ба.
✅ О́бе сестры́ верну́лись домо́й по́здно.
Both sisters came home late. (feminine → о́бе)
❌ В обе́их слу́чаях реше́ние бы́ло одина́ковым.
Incorrect — слу́чай is masculine, so the oblique form is обо́их, not the feminine обе́их.
✅ В обо́их слу́чаях реше́ние бы́ло одина́ковым.
In both cases the decision was the same. (masculine → обо́их)
❌ Я ви́жу пробле́му в о́ба вариа́нтах.
Incorrect — in the prepositional 'both' must take the oblique form and agree: в обо́их вариа́нтах.
✅ Я ви́жу пробле́му в обо́их вариа́нтах.
I see a problem in both options. (prepositional обо́их + вариа́нтах agree)
❌ Она́ держа́ла ча́шку о́бе рука́ми.
Incorrect — the instrumental needs the declined form обе́ими, agreeing with рука́ми.
✅ Она́ держа́ла ча́шку обе́ими рука́ми.
She held the cup with both hands. (instrumental обе́ими + рука́ми)
❌ О́ба бра́тья у́чатся в университе́те.
Incorrect — like два, о́ба takes the genitive SINGULAR in the nominative: о́ба бра́та, not the plural бра́тья.
✅ О́ба бра́та у́чатся в университе́те.
Both brothers study at university. (genitive singular бра́та)
Key Takeaways
- "Both" has two forms by gender: о́ба (masculine/neuter), о́бе (feminine) — о́ба бра́та, о́ба окна́, о́бе сестры́.
- In the nominative/accusative it works like два/две: it governs the genitive singular (о́ба бра́та, not *о́ба бра́тья).
- It declines, and the gender split survives: masculine/neuter обо́их, обо́им, обо́ими vs feminine обе́их, обе́им, обе́ими. Once declined, it agrees with the noun in case.
- Master the fixed phrase в обо́их слу́чаях and the pair с обе́ими рука́ми — they encode both traps: choosing the right gender and switching to the oblique form.
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- The Numeral Government Rule in DepthA2 — The single most important rule in Russian numbers, stated definitively for the nominative/accusative: a number ending in 1 (except 11) puts the noun in the NOMINATIVE SINGULAR (два́дцать оди́н дом); ending in 2, 3, 4 (except 12–14) → GENITIVE SINGULAR (два до́ма, три рубля́); ending in 0, 5–9, or being 11–14 → GENITIVE PLURAL (пять домо́в, двена́дцать книг). Plus where the rule comes from (the genitive singular is a fossilized dual), how adjectives agree inside a numeral phrase (два больши́х до́ма), and how compounds key on the final word (сто оди́н дом).
- Declining the Numerals ThemselvesB1 — Cardinal numerals are not frozen words — they decline through the cases. In the nominative and accusative the famous 1 / 2–4 / 5+ government rule decides the noun's case, but in the oblique cases (genitive, dative, instrumental, prepositional) the rule switches off entirely: the numeral and the noun simply AGREE in case. So о двух дома́х, с тремя́ друзья́ми, к пяти́ часа́м. This page gives the full declension tables for оди́н, два/две, три, четы́ре, пять–два́дцать, со́рок/девяно́сто/сто, the tens and hundreds, and shows that in a compound number EVERY word declines.
- Genitive: FormsA2 — The genitive (роди́тельный паде́ж) is one of the most-used and most-varied cases. The singular is tidy: masc/neuter -а/-я (стола́, окна́, музе́я), feminine -ы/-и (кни́ги, неде́ли, но́чи). The plural is the single hardest ending set in Russian — a three-way split between zero ending (often with a fleeting vowel: книг, о́кон, де́вушек), -ов/-ев (столо́в, музе́ев, отцо́в), and -ей (ноже́й, словаре́й, ноче́й). Learn the decision procedure, not a word list.
- Instrumental: FormsA2 — The instrumental (твори́тельный паде́ж) endings. Singular: masc/neuter -ом/-ем (столо́м, окно́м, мо́рем), feminine -ой/-ей (кни́гой, неде́лей) and the special feminine -ь → -ью (но́чью, две́рью). Plural: -ами/-ями for everyone (стола́ми, дверя́ми), with irregular людьми́, детьми́. The choice of -ом vs -ем turns on the spelling rule and stress.
- Numbers 11-100A1 — The teens (оди́ннадцать–девятна́дцать, built with -надцать), the tens (два́дцать, три́дцать, со́рок, пятьдеся́т…девяно́сто, сто), and compound numbers (два́дцать оди́н, три́дцать пять). The two irregular tens are со́рок (40) and девяно́сто (90). The all-important rule: in a compound number, the case of the noun is keyed to the LAST word — два́дцать оди́н рубль (nom. sg.), два́дцать два рубля́ (gen. sg.), два́дцать пять рубле́й (gen. pl.) — but the teens 11–14 ALWAYS take the genitive plural (оди́ннадцать рубле́й).