Before the EPO, the standard requirement is that the technical effects of a claimed invention and that of the closest prior art can be directly compared. Where a claimed invention is described through reference to “an unusual parameter” such comparison is difficult. Recent EPO Board of Appeal decision T 0555/18 is a caution against defining an invention by unusual parameters that make it difficult to compare the technical effect of the invention with that of the prior art. In this case, the EPO BoA found that the burden of proof shifted to the applicant for unusual parameters relied on for inventive step.