Cachets, which should not be confused with postmarks, are basically rubber stamps. Some fake cachets, cork cancels, surcharges and overprints were also used. Mail flown and delivered by the "Graf Zeppelin" airship received special cancellations and cachets. The lettres de cachets issued in the king's name were likewise nullified. It was provided in either crystal form or in cachets (capsules). There are official and private (independent of postal authorities) cachets. The others all had cachets for transmission to Knockaloe. Post offices may add cachets for special events such as a first flight.