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.