Remnants of the Dual: Hands, Eyes, Legs, Ears

Old Czech had three numbers, not two: singular, plural, and a dual for things that naturally came in pairs. The dual died out centuries ago — except in four words for body parts that come in twos, where its endings froze solid and are now obligatory standard Czech. These are ruka (hand/arm), noha (leg/foot), oko (eye), and ucho (ear). You cannot avoid them: they are among the most frequent nouns in the language, and using the "regular" plural endings on them sounds badly wrong to a native ear.

The good news is that there are only four words, and the body senses you need every day are exactly the ones that take the special forms. Learn these four little tables and you are done.

What the dual changed

The singular of all four is completely ordinary. ruka and noha decline like žena in the singular; oko and ucho decline like a hard neuter in the singular. The strangeness is concentrated in three plural cells: the genitive plural, the locative plural, and above all the instrumental plural, which keeps the historical dual ending -ma.

Two things to notice. First, ruka and noha make their genitive plural and locative plural identical (rukou, nohou) — a clear dual leftover, since the regular feminine ending would be a bare stem. Second, oko and ucho become feminine in the plural: the eyes and ears are oči and uši, and adjectives agree with them as feminine plurals (krásné oči, "beautiful eyes").

The -ma instrumental: the only place it is standard

The instrumental plural rukama, nohama, očima, ušima is the single most important thing on this page, because -ma is the genuine surviving dual ending — and this is the only spot in standard written Czech where you should write it. Everywhere else, an instrumental plural in -ma (like s kamarádama, před lidma) is colloquial Common Czech, fine in speech but wrong on paper. With these four body parts it is the opposite: -ma is the correct, formal, written form, and the "regular" rukami/nohami would be the error.

Crucially, the adjective that modifies them takes -ma too. Standard Czech says bosýma nohama (barefoot), holýma rukama (with bare hands), modrýma očima (with blue eyes). The adjective ending -ýma / -íma is itself a frozen dual, and it is standard here just as the noun is.

Body sense versus technical sense

Here is the elegant part. The dual forms belong to the body meaning. When oko and ucho mean something else, they are perfectly regular hard neuters with a plain plural — and the meaning splits cleanly down that line.

• oko = eye → plural oči (body); but oko = a loop, mesh, or drop of fat on soup → plural oka (regular neuter).
• ucho = ear → plural uši (body); but ucho = a handle (of a cup, a bag) or the eye of a needle → plural ucha (regular neuter).

So oka v síti are the meshes in a net, while oči are eyes in a face; ucha hrnce are the handles of a pot, while uši are the ears on your head. The form you choose announces the meaning.

That last sentence shows both systems side by side: ucha (handles, regular neuter) and rukama (hands, dual) in one breath.

Singular and the rest

For completeness: in the singular these four are unremarkable. ruka → bez ruky (genitive), v ruce (locative); noha → bez nohy, na noze; oko → do oka, v oku; ucho → bolí mě ucho, v uchu. The dual only wakes up in the plural.

Common mistakes

Key takeaways

• Four words keep dual forms: ruka, noha, oko, ucho — and only in the plural.
• The instrumental plural is rukama, nohama, očima, ušima, and this is the only standard written -ma instrumental in Czech; the adjective copies it (bosýma nohama, modrýma očima).
• oko and ucho turn feminine in the body plural: oči, uši.
• Non-body senses are regular neuters: oka (meshes, loops, drops), ucha (handles). The form picks the meaning.
• These are high-frequency, obligatory forms — umýt si ruce, bolí mě oči, na vlastní uši — not optional ornaments.