The properties used to characterise a bulk solid in a silo design have an enormous influence on the design. Indeed, most of the differences between the national standards mentioned above can be attributed to differences between material property definitions and their use. Silo designers and code writers alike face the challenge of selecting appropriate properties for a solid. Some codes define the properties of common materials, such as 'wheat' and 'coal', but the scatter for each can be very large. For solids not mentioned in the tables in codes, guesses or tests must be undertaken. This area has been one of considerable debate for many years.