exclave Since 1968, a peculiar rule has applied to the municipality of which is a German exclave completely surrounded by Swiss territory. Although Büsingen belongs to the German district of Konstanz, it is part of the Swiss customs area. For this reason, a vehicle registered to a citizen of Büsingen does not bear KN for Konstanz but BÜS, signifying to Swiss customs officers that this is in fact a domestic vehicle. There are about 700 cars with this area code, which makes Büsingen the smallest and least-populated registration precinct in Germany. List of Area Codes Stickers Registration plates become valid with the official seal of registration. This is a sticker of diameter, following the area code and bearing, in colours, the seal of the respective German with the name of the state and the issuing district authority added in print. Older stickers were monochrome, black on silver or white, and smaller (), depicting the seal of either the Bundesland or the city district. Vehicles used by federal institutions, such as , carry the German instead of a seal. The rear plate bears, above the official seal, the vehicle safety test sticker. This test is obligatory three years after the first registration, and every two years after that. The expiration date can be seen at one glance, as the sticker is attached with the month of expiration pointing upwards. The black marking, covering the sectors on either side of 12, thus makes it easy for the police to read the expiration month from a distance. Like the hand on a clock, the marking shows the position of a number on the face of a clock. The year when the next safety test is due is printed in the centre of the sticker and also indicated by the colour of the sticker which is repeated every six years. Between 1985 and 2010, a similar yet hexagonal sticker was applied to the front plate, certifying the emission test. Since that year, the emission test was incorporated into the safety test and was not performed separately any more, so the emission sticker became obsolete. All these stickers are specially treated to be easily transferred onto the licence plates, but hard to be removed without damaging the plate itself, making them relatively counterfeit-proof. The only licence plates which do not need to carry either seal are repeaters. These are obligatory when the original rear plate