According to T 256/87 all that has to be ensured is that the skilled person reading the specification will be able to carry out the invention in all its essential aspects and know when he is working within the forbidden area of the claims. The possibilities of indirect empirical investigation referred to in the specification were, in the board's view, an acceptable solution which sufficed to fulfil the requirements of Art. 83 EPC 1973 without undue burden. This decision was followed in T 387/01, T 252/02, T 611/02 and T 464/05. However, according to several decisions, the concept of 'forbidden area' was rather associated with the scope of the claims, i.e. Art. 84 EPC 1973, than with sufficiency of disclosure (see point 7.2 below).
In T 1886/06, the board emphasised that the above finding in T 256/87 could not mean, conversely, that if the claims used a term undefined under Art. 84 EPC 1973 the invention necessarily became impossible to carry out within the meaning of Art. 83 EPC 1973 in the absence, in the description or the common general knowledge of the skilled person, of concrete indications towards a possible definition; doubts as to the claims' reproducibility over their entire scope had to be substantiated by verifiable facts. A mere conjecture that their scope might extend to undisclosed variants was not enough.