A classification for prior art references established by WIPO in Standard ST. 14 for patent search reports. The X classification means that the reference entirely anticipates the invention. An X-reference is also called a knockout reference – a search report with an X-Reference will usually be regarded as a non-clean search report. However, the existence of an X-Reference does not mean that no patent can be secured – it may, for example, not knock-out all the dependent claims and it is also possible that the searcher may have fundamentally misunderstood the gist of the invention. See A-Reference, Y-Reference, E-Reference, and P-Reference.