The state of the art is held to comprise everything made available to the public by means of a written or oral description, by use, or in any other way, before the date of filing of the European patent application (Art. 54(2) EPC). The grounds on which the opposition is based may arise, for example, from circumstances of which the EPO was unaware when the patent was granted, e.g. prior use. Instances of public prior use or availability in any other way will typically be raised in opposition proceedings.