When the German Patent and Trade Mark Office grants a patent, the decision is not final at once. Under the Patent Act, jeder, anyone, may file an opposition within nine months after the grant is published. The opposition must be in writing and give reasons. One ground is that the invention is not patentable. Another is that the patent does not describe the invention clearly and completely enough for a skilled person to carry it out. The patent office then decides whether the patent is maintained, to what extent, or whether it is revoked.