Simple easy action to open and close, expand and collapse when racks are needed. Diesigned to help recognize 15 and 50mL tubes. Molded indexing for easy identification of tubes and vials. Racks can be stacked in open or closed position. Save space when racks are not in use. Available in individual 17mm hole diameter (15 ml) or 30 mm hole diameter (50 ml) or combination of both 17 mm & 30 mm (15 ml and 50 ml).