one-third

The compound 'one-third' is a transparent English construction combining the cardinal number 'one' with the ordinal 'third' to denote a specific fraction. 'One' traces back to Proto-Germanic ainaz and ultimately to Proto-Indo-European óynos, carrying the fundamental sense of a single unit. 'Third' derives from Old English 'þridda', from Proto-Germanic þridjô, which itself originates from the PIE root trey-, the source for the number three. The modern fractional sense emerged logically from the ordinal, designating one part out of three equal divisions. This compounding pattern is highly productive in English for forming fraction names, directly linking the count of parts to the whole they constitute.

Structure: one (1) + third (fractional part) - one: From Old English 'ān', meaning "one, a single". It functions as the cardinal number in the compound. - third: From Old English 'þridda', meaning "third in order". It functions as the ordinal number specifying the fractional denominator.