The compound "two-liter" is a modern English formation that transparently combines the cardinal number "two" with the metric unit "liter." The word "two" descends from the Proto-Indo-European root duwo-, which gave rise to the number across many Indo-European languages, signifying a fundamental concept of duality. "Liter" entered English via French in the late 18th century, tracing back to the Greek litra, a Sicilian unit of weight, which was later adopted into the metric system as a unit of volume. The compound's logic is purely descriptive, directly specifying a capacity of two liters, and it became a standardized commercial term, particularly for soft drink bottles, in the latter half of the 20th century.

Structure: two (number) + liter (unit) - two: From Old English twā, of Germanic origin; related to Dutch twee and German zwei, from an Indo-European root shared by Latin duo and Greek duo. Functions as a numerical quantifier. - liter: From French litre, from medieval Latin litra, from Greek litra (a unit of weight). Functions as a unit of measurement.