The compound adjective "two-hour" is a modern English formation that transparently combines the cardinal number "two" with the noun "hour." The number "two" has a deep Indo-European lineage, traceable to the Proto-Indo-European root dwóh₁, which gave rise to the Latin duo and, through Proto-Germanic, the Old English twā. The word "hour" entered English via Old French from Latin hōra, which itself was borrowed from Greek hōrā, originally meaning a season or a fixed period of time. The logic of the compound is purely descriptive and arithmetic, directly indicating a duration measured as two units of sixty minutes, with the hyphen serving to link the numeral and the noun into a single attributive adjective.

Structure: two (number) + hour (unit of time) - two: From Old English twā, the feminine and neuter form of twēgen (two). Of Proto-Germanic origin; related to Latin duo and Greek dyo. Functions as a cardinal number. - hour: From Old French hore, from Latin hōra, from Greek hōrā (season, time, hour). Functions as a noun denoting a unit of time.