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Crossover Drive
From WikiPedalia
Most tandems use crossover drive, in which the primary chain runs on the right, just like that of a solo, but the sync or timing chain is on the left side of the bicycle. The drive from the sync chain "crosses over" to the primary chain at the stoker's bottom bracket (or if the tandem has front drive, at the front bottom bracket.)
See also John Allen's article on tandem drivetrains..
