h₂eḱs-
“axle, axis”Source of Latin axis, English axle, axis
Root for the axle concept, yielding Latin axis and English axle — one of the earliest vehicle terms.
Discussion
The PIE root *h₂eḱs- (axle, axis, the rod around which rotation occurs) is one of the technologically diagnostic words in the reconstructed vocabulary — confirming that PIE speakers knew wheeled vehicles. The derivative form *h₂eḱ-s-lo- (axle as instrument) is treated separately.
Latin axis (axle, pivot, pole of the earth) gave English: axis (a line around which something rotates), axle (through Old Norse öxull), and axial. The astronomical use — the earth's axis — extends the mechanical concept to planetary rotation.
Greek áxōn (ἄξων, "axle") gave the anatomical/neurological term axon — a nerve fiber named for its axle-like shape, conducting signals along its length.
Sanskrit ákṣa- (axle) appears in Vedic texts describing chariots. Lithuanian ašìs (axle) and Old Church Slavonic osĭ confirm the Balto-Slavic reflexes.
The reconstruction of both *h₂eḱs- (axle) and *kʷekʷlo- (wheel) for PIE provides a minimum technological package: wheeled vehicles with axles. Combined with *yug-óm (yoke), the evidence places PIE in the early Bronze Age (~3500-3000 BCE).