oḱtṓw
“eight, the number eight”eight
PIE word for eight. Gives Latin octō, English "eight", Greek oktṓ, Sanskrit aṣṭā́.
Discussion
The Proto-Indo-European word *oḱtṓw is the cardinal numeral "eight" and presents one of the more intriguing puzzles of Indo-European numeral morphology. The word appears to contain a dual ending, leading many scholars to interpret it as meaning "two fours" or "two sets of four" — suggesting an earlier base-four counting system that was absorbed into the decimal system. This interpretation remains debated but is widely cited.
In Germanic, the initial vowel was retained and the palatal *ḱ became a velar, which then shifted under Grimm's Law. Old English eahta became modern English eight, with the characteristic -ght spelling representing an earlier velar fricative that has since fallen silent. German acht, Dutch acht, and Gothic ahtau are close cognates.
Latin octō "eight" preserves the root form with exceptional clarity and has been highly productive in English: October (originally the eighth month in the early Roman calendar beginning in March), octave, octagon, octopus (borrowed through Greek, literally "eight-footed"), and octane. The prefix octo-/octa- is standard in English scientific terminology.
Greek oktō "eight" produced the same set of derivatives, with octopus being the most familiar. Sanskrit aṣṭá "eight" (with the characteristic satem palatalisation), Lithuanian aštuoni, Old Irish ocht, Welsh wyth, and Old Church Slavonic osmĭ confirm the numeral across the full breadth of the family.
The dual-ending hypothesis — that "eight" originally meant "twice four" — is supported by the formal resemblance to dual morphology but remains unprovable. If correct, it would provide a remarkable window into Proto-Indo-European mathematical thinking, suggesting that the speakers built their higher numerals by combining lower ones, much as modern English constructs "fourteen" from "four" and "ten". Whether or not the dual analysis is accepted, *oḱtṓw stands as one of the best-attested words in the entire reconstructed lexicon.
Notes
Possibly a dual form meaning "two fours"; source of "octopus", "October"
Related Roots
English Words from *oḱtṓw
These modern English words descend from this root. Each page traces the full journey from PIE to present-day English.