For just these two plots a third material type has been added to complete the picture with regard to material types. This is material type C which covers materials that have very good permeability. To have such properties the material has to have a mean particle size above about 1 mm and for the particle size to be almost zero, so that it is effectively mono-sized. This group includes materials such as nylon and polyethylene pellets, and seeds and grains, such as peanuts. In terms of numbers of materials, this group is in a minority, compared with the other two, and so as not to confuse the issue