Two things arranged one behind the other, or working together as a pair.
From Latin 'tandem' meaning 'at length' or 'at last'; originally a joke Latin phrase used in 18th-century England for a two-horse carriage arrangement, eventually applied to any paired sequence.
Tandem bicycles seem impractical until you realize they're actually more efficient per person than two separate bikes—nature loves this arrangement, and you'll see it in everything from DNA strands to how migrating geese fly.
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