In the UK, a censorship notice issued by the Department of Defence to the media to prohibit the publication of information on matters alleged to be of national security. The system dates from 1922.
The top left trigram symbolizes summer, south, and heaven. The top right trigram represents autumn, west, and the moon. The bottom right trigram stands for winter, north, and the Earth. The bottom left trigram represents spring, east, and the sun. Effective date: 21 February 1984.