mey-
“to change, to exchange”change, exchange, migrate
Root for changing/exchanging, yielding Latin mutuus (mutual), mutare (to change), English mad.
Discussion
The PIE root *mey- (to change, to exchange, to go, to move) produced vocabulary for transformation, migration, and social exchange — connecting physical movement to conceptual change.
Latin mūtāre (to change) descends from the extended form *mey-tʰ- (treated separately) and gave English mutate, mutual, commute. The base root *mey- itself may underlie Latin migrāre (to move from one place to another — to change location), giving English migrate, emigrate, immigrate, and the combining form -migrant.
Latin mūnus (a duty, a gift, a public show — from *mey-nes, "an exchange-thing") gave English: municipal (mūnicipālis, pertaining to a town that exchanges duties with Rome), munificent (generously giving — abundant in exchange-gifts), immune (in-mūnis, "without duties" — exempt from the exchange of obligations), and commune (com-mūnis, "sharing duties together" — a community united by mutual obligation). The word community itself descends from communitas — the group defined by shared exchange.
The chain exchange → duty → gift → community encodes a social philosophy: a community IS a network of exchanges, and to be immune is to stand outside that network.
Sanskrit máyate ("he exchanges") confirms the Indo-Iranian reflex. Lithuanian maĩnas (exchange) provides Baltic attestation.
Notes
Pokorny 710. English mutual, mutate, commute, migrate, mad (changed in mind).