(of a crystal) that is a composite consisting of two (or sometimes more) parts which are reversed in orientation with respect to each other (typically by reflection in a particular plane)

cerrusite, pseudo-hexagonal crystals, usually twinned

a mass of twinned crystals

link (a town or district) with another in a different country or cause (two towns or districts) to be linked, for the purposes of friendship and cultural exchange

the Russian city of Kostroma is twinned with Durham