déḱm̥

The numeral *déḱm̥ ("ten") is among the most important items in PIE reconstruction, not only for its phonological regularity but for its role in establishing the centum-satem classification of IE languages. The treatment of the palatal *ḱ in this word — preserved as a velar in the "centum" languages and shifted to a sibilant in the "satem" languages — provides the primary diagnostic for this fundamental grouping. Latin decem ("ten") is the centum exemplar. From decem derive decimal, December (originally the tenth month of the Roman calendar), decimate ("to take a tenth"), dean (from decānus, "leader of ten"), and decade. The ordinal decimus gives dime (through Old French disme, "tenth part"). Greek déka (δέκα) shows the same centum treatment. The prefix deca- (decagon, decathlon, decibel) entered modern scientific vocabulary through Greek. The name Decapolis ("ten cities") illustrates the numeral in compound formation. Sanskrit dáśa shows the satem shift *ḱ > ś with complete regularity. The Persian dah and Avestan dasa confirm the Indo-Iranian development. Gothic taihun and Old English tēn (Modern English ten) show the Germanic reflex with Grimm's Law: *d > t and *ḱ > h (from the centum-merged velar *k). German zehn and Dutch tien continue the same line. Lithuanian dẽšimt and Old Church Slavonic desętĭ show the satem treatment *ḱ > s in Balto-Slavic. Old Irish deich and Welsh deg continue the Celtic reflex. The PIE decimal system — evidenced by the reconstructability of numerals from one to ten and beyond — is consistent with a base-ten counting system, itself suggestive of finger-counting. The numeral *déḱm̥ has been etymologically connected to the word for "hand" by some scholars (*deḱ- as "two hands" or "two sets of five"), though this analysis remains speculative.