The boards have consistently held that, where more than one document is cited as the closest prior art, the one which must be deemed the closest is that which provides the skilled person with the most promising springboard to the invention, i.e. the one starting from which the subject-matter of the invention is rendered most obvious (T 656/90, T 824/05, T 1755/07).
The starting point for the assessment of inventive step should be one which is at least "promising", in the sense that there was some probability of a skilled person arriving at the claimed invention. However, when applying these principles, care must be taken to avoid an ex-post-facto-approach. A skilled person endeavouring to arrive at a simple construction is unlikely to begin by using prior art relating to an exceptional embodiment with a complex mechanism, and then to omit this mechanism from the invention (T 871/94).