Under the EPC 1973, the Legal Board allowed correction of state designations under R. 88, first sentence, EPC 1973 (now R. 139 EPC), early on (J 8/80, OJ 1980, 293; J 12/80, OJ 1981, 143; J 3/81, OJ 1982, 100; J 21/84, OJ 1986, 75). Shortly afterwards the rather strict principles developed in these decisions were also applied in cases where correction of priority declarations was at stake. Although a mistake correctable under R. 88, first sentence, EPC 1973 could be an incorrect statement or the result of an omission, all the previous cases related to omitted priority declarations (J 3/82, OJ 1983, 171; J 4/82, OJ 1982, 385; J 14/82, OJ 1983, 121; J 11/89 and J 7/90, OJ 1993, 133).

In the four decisions J 3/91 (OJ 1994, 365), J 6/91 (OJ 1994, 349), J 9/91 and J 2/92 (OJ 1994, 375), the Legal Board refined the principles to be applied to corrections of priority declarations. In J 6/91 it analysed and summarised the previous case law, pointing out that the applicant had to prove a mistake, i.e. that the document filed with the EPO did not express the true intention of the person on whose behalf it was filed. In the earlier decisions the burden of proof on the applicant was defined as a heavy one. In J 9/91, however, the board took the view that the omission of a priority declaration would, in nearly every case, be an error. Thus, as a general rule, there was no need in cases of this kind to require special evidence to discharge the burden on the applicant of proving that a mistake had been made.