Learn Icelandic: Margir klæðskerar hafa saumað pils sem eru sextíu sentímetra á lengd. - Many tailors have sewn skirts that are sixty centimeters in length.

In Icelandic, it is very common to express physical dimensions using the structure [Measurement] + á + [Dimension Noun]. Instead of using a descriptive adjective to say the skirts are "long," this phrasing literally translates to "60 centimeters on length." The noun lengd is in the accusative case here, governed by the preposition á.

What case is the measurement sentímetra in, and why?

It is in the accusative plural. When stating a specific measurement right before the á + [dimension noun] structure, Icelandic uses the "accusative of measure" to express the exact amount, distance, or size.

Does the number sextíu decline to match the accusative plural sentímetra?

No. While the numbers 1 through 4 in Icelandic decline for gender and case, all numbers ending in -tíu (like sextíu, meaning sixty) are completely indeclinable. They stay exactly the same regardless of the grammatical role they play in the sentence.

Since the word pils can mean either "skirt" or "skirts," how do we know it is plural here?

Pils is a neuter noun, and like many single-syllable neuter nouns, its plural form is identical to its singular form in the nominative and accusative cases. We can tell it is plural here by looking at the relative clause that follows it: the pronoun sem acts as the subject for the plural verb eru (are).