Letterboxing is an outdoor hobby that combines elements of orienteering, art, and puzzle solving. Letterboxers hide small, weatherproof boxes in publicly accessible places (like parks) and distribute clues to finding the box in printed catalogs, on one of several web sites, or by word of mouth. Individual letterboxes contain a notebook and a rubber stamp, preferably hand carved or custom made. Finders make an imprint of the letterbox's stamp in their personal notebook, and leave an impression of their personal signature stamp on the letterbox's "visitors' book" or "logbook" — as proof of having found the box and letting other letterboxers know who has visited. Many letterboxers keep careful track of their "find count".
The origin of letterboxing can be traced to Dartmoor, Devon, England in 1854. William Crossing in his Guide to Dartmoor states that a well known Dartmoor guide (James Perrott) placed a bottle for visiting cards at Cranmere Pool on the northern moor in 1854. From this hikers on the moors began to leave a letter or postcard inside a box along the trail (sometimes addressed to themselves, sometimes a friend or relative)—hence the name "letterboxing". The next person to discover the site would collect the postcards and post them. In 1938 a plaque and letterbox in Crossing's memory were placed at Duck's Pool on southern Dartmoor.
The first Dartmoor letterboxes were so remote and well-hidden that only the most determined walkers would find them, allowing weeks to pass before the letter made its way home. Until the 1970s there were no more than a dozen such sites around the moor, usually in the most inaccessible locations. Increasingly, however, letterboxes have been located in relatively accessible sites and today there are thousands of letterboxes, many within easy walking distance of the road. As a result, the tradition of leaving a letter or postcard in the box has been forgotten.
Membership of the "100 Club" is open to anyone who has found at least 100 letterboxes on Dartmoor. Clues to the locations of letterboxes are published by the "100 Club" in an annual catalogue. Some letterboxes however remain "word of mouth" and the clues to their location can only be obtained from the person who placed the box. Some clues may also be found in other letterboxes or on the Internet, but this is more commonly for letterboxes in places other than Dartmoor, where no "100 Club" or catalogue exist.