On Casters, Removable Dividers. Use with the Panel Bed, Low Poster Bed, Bunk Bed or Bunk Bed with Extension under the Bed Rails. Takes a standard foam trundle mattress (max size: 73x38x8) for use as an extra bed or 2 removable dividers (included) allow for organized underbed storage. Requires bed rails. Rails must be in high lock position.