The ruler is a vital tool in the accurate construction of cardboard models. There are three common materials used in ruler manufacture, wood, metal and plastic. Wood rulers can be discarded because they are generally inaccurate and offer few advantages. A metal ruler is a hard wearing tool well able to withstand the daily wear of scalpel blade and scissors. It is, however, opaque, and expensive. For this reason we favour cheap and cheerful plastic rulers as available from the local shop. Cheap, transparent and reasonably long lasting as long as you are careful with your blade.