Simple fractional numbers 1/3, 1/4 etc. are mostly named systematically using compound words consisting of the noun osa (part) preceded by an ordinal numeral: kolmasosa, neljäsosa etc. (literally “third part”, “fourth part”). However, special derived words kolmannes : kolmanneksen, neljännes, viidennes etc. are alternatively used for numbers 1/2, 1/3, …, 1/9, and 1/100. These words can be regarded as derived from cardinal numerals with the nnes : nekse- suffix or, perhaps simpler, as derived from the ordinal numeral stems with the s : kse- suffix.
The word for number 1/2 is puoli : puolen.
For the number 1/10, the words kymmenennes : kymmenenneksen and the less regular kymmenes : kymmeneksen are sometimes used, as an alternative to kymmenesosa. The word kymmenys : kymmenyksen has special meanings: it means a decimal digit in the decimal part or, in plural (kymmenykset) in religious use, tithes.
Numbers such as 2/5 are expressed systematically: kaksi viidesosaa (literally, “two fifth parts”) or, less often, kaksi viidennestä.
The number 1½ can be read as yksi ja puoli, but more idiomatically and more often, it is read as puolitoista, where the first part is inflected, e.g. the inessive is puolessatoista. Similar expressions for other numbers, such as puolikolmatta for 2½, are hardly used in modern language any more. Instead, simple expressions like kaksi ja puoli are used. In them, both parts are inflected, e.g. kahdessa ja puolessa.