The compound adjective 'ten-hour' is a transparent combination of two ancient roots. The first element, 'ten', descends directly from the Proto-Indo-European word for the number, 'déḱm̥t', which remained remarkably stable through Proto-Germanic into Old English. The second element, 'hour', has a more complex journey, originating from the Greek 'hṓrā', which broadly meant a fixed period or season. This was borrowed into Latin as 'hōra', specifically denoting a 24th part of a day, and then passed into English via Old French. The modern compound logically fuses these elements, with 'ten' quantifying the specific duration expressed by 'hour', creating a precise descriptor for a decade of hours. This pattern of number-unit compounding is highly productive in English, directly reflecting the literal meanings of its constituent parts.

Structure: ten (number) + hour (unit of time) - ten: From Old English 'tēn', 'tīen', from Proto-Germanic 'tehun', from Proto-Indo-European 'déḱm̥t'. Functions as a numerical root. - hour: From Old French 'hore', from Latin 'hōra', from Greek 'hṓrā' (season, time, hour). Functions as a noun root denoting a unit of time.