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In the first edition of The International Classification of Headache Disorders there was no appendix. This time an appendix is added which, we hope, will be used in several ways.

The primary purpose of the appendix is to present research criteria for a number of novel entities that have not been sufficiently validated by research studies. However, the experience of the experts in the Headache Classification Subcommittee and publications of variable quality suggest that there are a number of diagnostic entitities that are believed to be real but for which further scientific evidence must be presented before they can be formally accepted. Therefore it is anticipated that a number of the disorders now in the appendix will move into the main body of the classification next time the classification is revised.

In a few places we present an alternative set of diagnostic criteria to those in the main body of the classification. This is again because clinical experience and a certain amount of published evidence suggest that this may be a good idea, but the subcommittee still does not feel that the evidence is sufficient to change the main classification. This is, for example, the case for the accompanying symptoms of migraine without aura. The alternative diagnostic criterion D in the appendix is easier both to understand and to apply, but not yet sufficiently validated.

Finally, the appendix is used as a first step in eliminating disorders included as diagnostic entities in the first edition because of tradition but for which sufficient evidence has still not been published.