eighteen-month

The compound adjective "eighteen-month" is a modern English formation following a straightforward numeral-noun pattern to denote duration. Its first element, "eighteen," traces back to Old English 'eahtatēne', literally meaning "eight and ten," a common Germanic construction for numbers between 13 and 19. The second element, "month," originates from Old English 'mōnath', derived from Proto-Germanic 'mēnōth-', which is directly tied to the moon and its cyclical phases. The logic of the compound is transparent: it directly combines the measure of quantity ("eighteen") with the unit of time ("month") to create a specific temporal descriptor, reflecting English's analytic tendency to form clear, compounded modifiers.

Structure: eighteen (numeral) + month (noun) eighteen: From Old English 'eahtatēne', a compound of 'eahta' (eight) and 'tēne' (ten). Functions as a numeral root. month: From Old English 'mōnath', related to 'mōna' (moon), indicating a lunar cycle. Functions as a noun root.