The first point that I would like to make about the differences between a structured and unstructured vocabulary is that the question is very much related to vocabulary size and this has certainly in the past generated a good deal of controversy in Aslib co‐ordinate indexing circles. Mr Snel and Mr Boyd have been two of the leading advocates of a restricted vocabulary and they argue that this keeps both indexing and searching simple. They also argue that it allows for human intelligence to play a much bigger part in the search process by allowing the sifting of search output to be done by the inquirer and not the system. It is also assumed, probably correctly, that the bigger the vocabulary the more noise a system will produce. The reason that I believe that the question of structured vocabularies and vocabulary's size are related is simply that if one does employ structure, i.e. a classificatory element of some sort, then the practical result of this is a bigger vocabulary. May I interject with a quotation from Cyril Cleverdon, who, to my way of thinking, summed up the question of vocabulary size in a conference that the Co‐ordinate Indexing Group organized some considerable while ago: ‘Much of the disagreement has been due to the fact that speakers were arguing from different view points. If it was found that a particular system operated satisfactorily for a certain organization, then obviously there was no need to find fault with it. However, it was unjustifiable to make claim as speakers had done that the same system would necessarily operate satisfactorily in all other situations. There were obvious differences when one organization was more interested in having a good recall ratio and was relatively unconcerned about relevance, whilst another organization was more interested in having a good relevance ratio. Certain indexing devices were available which could bring about either of these situations but it was unlikely that any of the operating systems discussed would satisfactorily meet both requirements.’