h₂eḱ-s-lo-
“axle, turning rod”axle, axis
Extended form of *h₂eḱ- giving Latin axis, English axle, axis.
Discussion
The PIE form *h₂eḱ-s-lo- (axle, the rod around which a wheel turns) derives from the root *h₂eḱs- (axis, axle), which itself may connect to *h₂eḱ- (sharp, pointed — the axle as a pointed rod). Like *kʷekʷlo- (wheel), the axle-word is archaeologically significant: its reconstruction confirms that PIE speakers knew wheeled technology.
Latin axis (axle, pivot, pole of the earth) continues the root and gave English: axis (a line around which something rotates, or a political alliance — the Axis powers of WWII were named for the Rome-Berlin "axis" around which European politics was said to turn), axle (through Old Norse öxull, from the same PIE root but via Germanic rather than Latin), and axial (pertaining to an axis).
Greek áxōn (ἄξων, "axle, axis") preserves the form in the Hellenic branch and gave the mathematical and anatomical term axon (a nerve fiber that conducts signals along its length, named for its axle-like shape).
Sanskrit ákṣa- (axle, axis) confirms the Indo-Iranian reflex. The word appears in Vedic texts describing chariots — the chariot's axle being one of the critical engineering elements that determined its performance in battle and racing.
Old English eax (axle) preserves the native Germanic reflex, though this form was later displaced by the Norse-derived axle (from öxull). Lithuanian ašìs (axle) and Old Church Slavonic osĭ (axle) provide the Balto-Slavic cognates.
The reconstruction of both *kʷekʷlo- (wheel) and *h₂eḱs-lo- (axle) for PIE provides a minimum technological package: the proto-speakers knew wheeled vehicles with axles. Combined with *yug-óm (yoke) — confirmed by Latin iugum, Greek zygón, Sanskrit yugá- — the evidence points to a PIE culture that used yoked, wheeled transport. This trio of technical terms (wheel + axle + yoke) is one of the strongest arguments for dating PIE to the early Bronze Age (c. 3500-3000 BCE) and placing the homeland in the Pontic-Caspian steppe, where the earliest evidence of wheeled vehicles appears in the archaeological record.