1. In the cases referred to in Article 6, Article 15 Article 32 and Article 32 and Article 34 of the Convention, as the case may be, insurance periods shall be added together in accordance with the following rules:
(a) to the insurance periods completed under the provisions of the legislation of one participating State shall be added the insurance periods completed under the provisions of the legislation of any other participating State to the extent necessary to make up the total insurance period required under the provisions of the legislation of the first-mentioned participating States for the acquisition or maintenance of entitlement to benefit provided always these periods do not overlap. In the case of invalidity, old age, or survivors' benefits to be paid by the institutions of 2 or more participating States in accordance with the provisions of Article 16 of the Convention, each of the institutions concerned shall separately add together all the insurance periods completed by the person concerned under the provisions of the legislation of all the Participating States to which he has been subject;
(b) where a period of compulsory insurance completed in accordance with the legislation of one participating State coincides with a period of voluntary insurance completed under the provisions of the legislation of another participating State, the first only shall be taken into account;
(c) where the time at which certain insurance periods were completed under the provisions of the legislation of a participating State cannot be accurately determined, such periods shall be presumed not to overlap with periods completed under the legislation of another Participating State and shall be taken into account as may be necessary;
(d) where, according to the legislation of one participating State, certain insurance periods are taken into account only if they have been completed within a specified time, the institution which applies this legislation shall take into account only periods completed under the provisions of the legislation of another participating State as have been completed within the same specified time.
2. Where under the provisions of the legislation of a participating State a social security scheme falling within the scope of the Convention takes account of insurance periods in respect of schemes not falling within the scope of the Convention such insurance periods shall be considered by the participating States to be taken into account for the purpose of adding together.