The Letterboxing reference article from the English Wikipedia on 24-Apr-2004
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Letterboxing

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1. Letterboxing is an outdoor hobby which began on the moors of Dartmoor, England. Letterboxers hide a small weatherproof box in publicly accessible places and post clues to its location. Clues to letterboxes on Darmoor are traditionally distributed in print format in the Dartmoor 100 Club's regularly-updated catalogue; in the U.S., clues are published on the Letterboxing North America and other web sites. Interest in letterboxing in the United States is generally considered to have started with a feature article in the April 1998 Smithsonian Magazine.

Further information:

See also: Geocaching, Orienteering

2. "Letterboxing" is also a term used to describe the process of transferring a widescreen image to video in a widescreen ratio. See letterbox.