Where, as disclosed in the application, the several inventions claimed are related, and such related inventions are not patentably distinct as claimed, restriction under 35 U.S.C. 121 is never proper (MPEP § 806.05). If applicant voluntarily files claims to such related inventions in different applications, double patenting may be held.

Where the inventions as claimed are shown to be independent or distinct under the criteria of MPEP § 806.05(c) - § 806.06, the examiner, in order to establish reasons for insisting upon restriction, must explain why there would be a serious search and/or examination burden on the examiner if restriction is not required. In order to demonstrate a serious search burden, the examiner must show by appropriate explanation one of the following:

1. (A) Separate classification thereof: This shows that each invention has attained recognition in the art as a separate subject for inventive effort, and also a separate field of search. Patents need not be cited to show separate classification.
2. (B) A separate status in the art when they are classifiable together: Even though they are classified together, each invention can be shown to have formed a separate subject for inventive effort when the examiner can show a recognition of separate inventive effort by inventors. Separate status in the art may be shown by citing patents which are evidence of such separate status, and also of a separate field of search.
3. (C) A different field of search: Where it is necessary to search for one of the inventions in a manner that is not likely to result in finding art pertinent to the other invention(s) (e.g., searching different classes/subclasses or electronic resources, or employing different search queries), a different field of search is shown, even though the two are classified together. The indicated different field of search must in fact be pertinent to the type of subject matter covered by the claims. Patents need not be cited to show different fields of search.

Where, however, the classification is the same and the field of search is the same and there is no clear indication of separate future classification and field of search, no reasons exist for dividing among independent or related inventions.

To demonstrate serious examination burden separate from a serious search burden, the examiner must show by appropriate explanation that the inventions are likely to raise serious examination issues, such as non-prior art issues under 35 U.S.C. 101, pre-AIA 35 U.S.C. 112, first paragraph and/or 35 U.S.C. 112(a). In this situation, a serious examination burden may exist where issues relevant to one invention are not relevant to the other invention.