One after another; -- said especially of horses harnessed and driven one before another, instead of abreast.
- A two-wheeled carriage drawn by horses harnessed one before the other.
- A team of carriage horses harnessed in single file.
- A tandem bicycle.
- An arrangement of two or more persons or objects placed one behind the other: driving horses in tandem.
- Having two identical components arranged one behind the other: a tandem axle.