Stoppers, also known as bungs, are used to give an airtight seal to laboratory flasks and test tubes by fitting tightly inside a container's neck, preventing material from spilling out. Rubber stoppers are often sized with a stopper number or stopper size, using the stopper's top and bottom diameter dimensions. Sizing can vary among manufacturers. Stoppers are typically designed with a conical shape, but straight and flanged options are also available. Rubber stoppers are available in a solid design or with holes, also known as positions, present on the stopper's top. Positions are used to allow easy access to the contained material for pipettes, tubing, or other equipment without removing the stopper. These products are available in a variety of shapes, materials, and colors.