A pattern must serve the purpose of making a mould which, when filled with molten metal, will produce a casting of a certain form and size. It would at first thought appear that to serve this purpose the pattern should be made an exact model of the desired casting. This is true only for some small simple patterns. There are some styles of patterns, or rather substitutes for patterns, which bear little if any resemblance to the moulds to be shaped from them.

The essential features which enter into the making of patterns and which are made necessary by considerations of their practical use are outlined in paragraphs 237 to 241 inclusive.