The compound "third-quarter" is a modern financial and calendrical term built from two ancient numerical concepts. "Third" originates from the Proto-Indo-European root for "three," evolving through Germanic to denote the ordinal position. "Quarter" stems from the Latin quartus, meaning "fourth," reflecting the division of a whole into four equal parts. The logic of the compound is transparent: it literally means "the third of four equal sequential periods," typically applied to a year or fiscal cycle. This construction exemplifies how English combines simple, inherited numerical terms to create precise terminology for time management and economic reporting.

Structure: third + quarter - third: From Old English þridda, from Proto-Germanic þridjô, from Proto-Indo-European tritiyos. Functions as an ordinal number. - quarter: From Old French quartier, from Latin quartarius ("fourth part"), from quartus ("fourth"). Functions as a noun denoting a division or part.