The -u vs -a Genitive of Inanimate Masculines

Every hard inanimate masculine noun — the hrad type — has to make one choice in the genitive singular that the rest of its paradigm never forces on it: does it take -u or -a? Do hradu but do lesa; bez vlaku but bez chleba. The two endings are not interchangeable, the choice is fixed for each word, and getting it wrong (*do lesu, *bez chleba → *bez chlebu) is one of the most audible errors a foreigner makes, because these are everyday words a native says hundreds of times a week. This page is about how the split works, which words fall on which side, and why there is no shortcut that fully saves you from learning them.

The default is -u — but it is only a default

Statistically, -u wins. The large majority of hard inanimate masculines take -u in the genitive singular, and a noun you have never met before is far more likely to be a -u word than an -a word. So when you are guessing, guess -u.

The trouble is that the -a group, though a minority, is packed with extremely high-frequency words. So even though "most nouns take -u" is true, you will reach for the -a words constantly — which means you cannot get away with simply applying the majority rule and forgetting about it.

The -a group: a closed, learnable list

The nouns that take -a form a historically determined set. It is not productive — new borrowings (e-mail, web, tablet) all join the -u camp — so the -a group is closed and finite. You can simply learn it. Here are the most important members, grouped so they are easier to fix in memory:

Notice rybníka: the -a/-u split is not the same question as the velar-stem rule that governs the locative. A noun can end in a velar (k, h, ch) and still take -a in the genitive — rybník → rybníka — even though that same noun is forced into -u in the locative (na rybníku). The two splits are independent; do not let the locative's velar rule leak into the genitive.

The tendencies — real but leaky

There is no rule that decides the split for you with certainty, but there are genuine tendencies that improve your guesses. Be clear-eyed about them: they are statistical leanings, not laws, and every one has exceptions.

• Old, native, concrete words lean -a. The -a ending is the older inheritance, so the nouns that have been in the language longest — les, svět, chléb, kostel, dvůr — are exactly the ones that kept it. Recent and abstract words took the newer -u.
• Many monosyllables lean -a, again because they tend to be old core vocabulary: les, svět, dvůr, sýr, sklep. But plenty of monosyllables still take -u (byt → bytu, most → mostu, vlak → vlaku), so this is a hint, not a test.
• Abstract, technical, and foreign nouns take -u almost without exception: názor → názoru (opinion), systém → systému, autobus → autobusu, víkend → víkendu.

Place names: a productive -a corner

Foreign and many Czech masculine place names are a notable pocket where -a is alive and even spreading. City and country names that end in a consonant frequently take -a in the genitive: Londýn → Londýna, Berlín → Berlína, Řím → Říma (Rome), Egypt → Egypta. This matters because you reach for the genitive of place names constantly — after do (to/into), z/ze (from), and u (near).

Not every masculine place name takes -a — some take -u (z Londýnu is heard regionally and historically, but z Londýna is standard today) — so once again this is a tendency, not a guarantee. For the wider behaviour of place names see Czech place names.

When both endings are possible

A handful of nouns genuinely allow both endings, sometimes with a difference in meaning or register and sometimes as free variants:

• sýr → sýra / sýru — both are heard; sýra is the more traditional.
• tábor → tábora (camp) — the genitive of the common noun; the city Tábor also declines this way.
• Several abstract/concrete pairs split by sense: a word used concretely may lean -a, the same word used abstractly -u.

When a noun allows both, you cannot be wrong by picking either standard variant — a relief after all the words where only one is correct.

Common mistakes

The pattern in these errors runs both ways, which is what makes the split hard: English speakers tend to pick -u everywhere (because it is the default they were taught first) and so trip on the -a words like les and chléb; then, once they learn that -a exists, they sometimes overcorrect and force -a onto borrowings like autobus. Neither blanket strategy works. The only reliable method is to learn the genitive together with the noun.

Key takeaways

• The hard inanimate masculine genitive singular splits between -u (the majority and the safe default) and -a (a closed, high-frequency group).
• The -a group is old, native, often concrete or monosyllabic: les → lesa, svět → světa, chléb → chleba, kostel → kostela, dvůr → dvora, domov → domova, oběd → oběda, rybník → rybníka.
• Abstract, technical, and borrowed nouns take -u almost without exception: názor → názoru, autobus → autobusu, víkend → víkendu.
• Many masculine place names take -a: Londýn → Londýna, Berlín → Berlína, Egypt → Egypta.
• The genitive -a/-u split is independent of the locative -ě/-u split — a noun can take -a in the genitive but -u in the locative (rybníka but na rybníku). See the locative split.
• There is no fully reliable rule. Learn the genitive as part of each word.