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https://en.wikipedia.org/wiki/Temple_garment

“A temple garment, also referred to as garments, the garment of the holy priesthood,[2][3][4] or Mormon underwear,[5] is a type of underwear worn by adherents of the Latter Day Saint movement after they have taken part in the endowment ceremony. Garments are worn both day and night and are required for any adult who previously participated in the endowment ceremony to enter a temple.[6] The undergarments are viewed as a symbolic reminder of the covenants made in temple ceremonies and are seen as a symbolic and/or literal source of protection from the evils of the world.”

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“Temple garments are sometimes derided as “magic underwear” by non-Mormons, but Mormons view this terminology to be both misleading and offensive.”