mey-tʰ-
“to exchange, trade, change”mutual, commute, permute, immutable
Extended form of *mey- giving Latin mūtāre, English mutual, commute, permute, immutable.
Discussion
The PIE root *mey-tʰ- (to exchange, to change, to trade) produced the vocabulary of transformation and reciprocity — a word family that connects commercial trading with biological mutation and physical commuting, all unified by the concept of exchange.
Latin mūtāre (to change, to alter, to exchange) continues the root and generated one of English's most productive derivative families: mutate (to change form — the biological sense was added by Hugo de Vries in 1901), mutation (a change), mutual (shared between parties — literally "exchanged," what goes back and forth), commute (com-mūtāre, "to change together" — originally to exchange one penalty for another, later to travel back and forth between home and work), permute (per-mūtāre, "to change through" — to rearrange), transmute (to change across — to transform one substance into another, the alchemist's dream), and immutable (not changeable, fixed).
The commuter's daily journey is, etymologically, an exchange: you commute your time and effort in one place for wages earned in another. The word's shift from "exchange a penalty" to "travel regularly" occurred in 19th-century America when rail passengers exchanged (commuted) multiple single tickets for a season pass.
Sanskrit méthati ("he exchanges, he alternates") and the related mithu- ("changing, alternating") preserve the Indo-Iranian reflex. The Vedic concept of exchange appears in ritual contexts: the sacrifice is an exchange between humans and gods.
Old Irish mith (exchange) and Gothic maidjan (to change, to falsify) provide Celtic and Germanic attestation.
The root's semantic coherence — exchange, change, transformation — encodes a conceptual equation: all change IS exchange. To change form is to exchange one state for another. To commute is to exchange locations. To mutate is to exchange one genetic letter for another. The PIE speakers who named trading as "exchanging" generated a root that their descendants would apply to genetics, chemistry, and daily transportation.