*1854 James Perrot left a calling card in a glass bottle at Cranmere Pool, Dartmoor and invited friends to find it.
*1888 A small tin box replaced the original bottle. Visitors left self-addressed postcards. The next person to visit the letterbox (except if it was a same-day visitor) would retrieve the postcards and mail them back from their hometown. And so the activity gets named "Letterboxing".
*1976 Tom Gant created a guide map pinpointing the fifteen letterboxes in existence, at which point letterboxing began to boom in a big way.
*1980s Commercial rubber stamps and a notebook are used instead of calling cards or postcards. Clues are published in a clue catalogue or by Word of Mouth (WOM).
*1998 Smithsonian publishes an article called "They Live and Breathe Letterboxing"
*1998 April 24. Max Patch planted by an orienteer in Hot Springs, NC. The first box planted in the U.S.
*1998 April 30. Prayer Rock is planted in Vermont. This is the first hand carved stamp planted in the United States (there is some dispute as to whether Max Patch or Prayer Rock should be classified as the first).
*1999 Letterboxing North America, the original source of online letterbox clues, is born.
*2001 Feb 21. Probably Canada's first letterbox: Beaver Mountain Provincial Park Letterbox, Nova Scotia, Canada (Antigonish) Adopted by Jiggs in 2006
*2002 March 16. First postal letterbox.
*2002 LB hiders can post and maintain their own clues on the LbNA site.
*2004 The Atlas Quest letterboxing website is born!

History of Letterboxing (AQ)
Cranmere Day (AQ)
First State Letterboxes (Mark & Sue)
A Short History of Letterboxing on Dartmoor (Silent Doug)