The invention relates to a method and a flat key for unlocking a frozen-up cylinder lock, in particular a motor-vehicle lock. A flat key 1 is produced from Cu or Ag or from one of the alloys thereof, and its handle 2 is of a size which ensures a good transfer of heat from the hand to the key. By virtue of the heat transmitted from the hand to the key handle, when the key is inserted de-icing takes place. <IMAGE>
