Beardsley's approach was refined by Stephen N. Thomas, whose 1973 book "Practical Reasoning In Natural Language" introduced the term "linked" to describe arguments where the premises necessarily worked together to support the conclusion. However, the actual distinction between dependent and independent premises had been made prior to this. The introduction of the linked structure made it possible for argument maps to represent missing or "hidden" premises. In addition, Thomas suggested showing reasons both "for" and "against" a conclusion with the reasons "against" being represented by dotted arrows. Thomas introduced the term "argument diagram" and defined "basic reasons" as those that were not supported by any others in the argument and the "final conclusion" as that which was not used to support any further conclusion.