swéḱs
“six, the number six”six, number
PIE word for six. Gives Latin sex, English "six", Greek héx, Sanskrit ṣáṣ.
Discussion
The Proto-Indo-European word *swéḱs is the cardinal numeral "six" and is one of the core numerals reconstructable with high confidence across the Indo-European family. The initial cluster *sw- and the palatal stop *ḱ underwent characteristic transformations in each branch, producing forms that at first glance look quite different but are connected by regular sound correspondences.
In Germanic, the palatal *ḱ became a velar *k and then, by Grimm's Law, the fricative *h (later lost or reduced). The initial *sw- was simplified to *s-. Old English siex became modern English six. German sechs and Dutch zes are close relatives. The transparency of the connection between six and *swéḱs is somewhat obscured by these changes but remains clear to the trained eye.
Latin sex "six" preserves the root well and has been enormously productive in English borrowings: semester (literally "six months"), sextet, sextant, and Sistine (from sextus "sixth", as Pope Sixtus). The Latin ordinal sextus also gave rise to various calendar and musical terms. Greek hex "six" (with regular loss of the initial s-) produced hexagon, hexameter, and hexapod, all staples of English technical vocabulary.
Sanskrit ṣáṣ "six", Avestan xšvaš, Lithuanian šeši, Old Irish sé, Welsh chwech, and Old Church Slavonic šestĭ all descend from the same proto-form. The variety of surface forms — six, hex, sex, šeši — demonstrates how regular sound laws can produce apparently unrelated words from a single source.
Like the other low numerals, *swéḱs belongs to the most ancient and stable layer of the Indo-European vocabulary. The numeral system as a whole provides some of the strongest evidence for the genetic unity of the Indo-European family, and six, sitting comfortably in the middle of the count, has proved as durable as any.
Notes
Source of "hexagon", "semester" (six months)
Related Roots
English Words from *swéḱs
These modern English words descend from this root. Each page traces the full journey from PIE to present-day English.