A trundle bed (or truckle bed) is a low, wheeled bed that is stored under a twin/single bed and can be rolled out for use by visitors or as just another bed.
A pop-up trundle bed may be raised to meet the height of the normal bed, effectively creating a wider sleeping surface when positioned side-by-side.
The trundle bed was invented sometime in the early 1600s.
Its name comes from the old word "tryndel" which was an old English word for wheel or roller. (citation: the craftsman blog)
Trundle beds were first used by royalty as a method to make sure their servants would always be close by so they could call on them for any given task. (citation: the craftsman blog) However, since then their usage has changed from such a royal type.
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See also
Trundle (disambiguation)