There are two popular competition word lists used in various parts of the world: TWL and SOWPODS. The North American 2006 Official Tournament and Club Word List, Second Edition (OWL2), became official for use in American, Canadian, Israeli and Thai club and tournament play on March 1, 2006 (or, for school use, the bowdlerized Official Scrabble Players Dictionary, Fourth Edition (OSPD4)). Early printings of OWL2 and OSPD4 must be amended according to corrigenda posted at the National Scrabble Association web site. North American competitions use the Long Words List for longer words.
The OWL2 and the OSPD4 are compiled using four (originally five) major college-level dictionaries, including Merriam-Webster (10th and 11th editions, respectively). If a word appears (or historically appeared) in at least one of the dictionaries, it is included in the OWL2 and the OSPD4, unless the word has only an offensive meaning, in which case it is only included in the OWL2. The key difference between the OSPD4 and the OWL2 is that the OSPD4 is marketed for "home and school" use, and has been expurgated of many words which their source dictionaries judged offensive, rendering the Official Scrabble Players Dictionary less fit for official Scrabble play. The OSPD4 is available in bookstores, whereas the OWL2 is only available from the National Scrabble Association's retail website wordgear.com (as of July 2009, NSA membership is no longer required to purchase the OWL).
In all other countries the competition word list is the Tournament and Club Word List (Collins) published in May 2007 (SOWPODS), which lists all words of from 2 to 15 letters and is thus a complete reference. This list contains every word in the OWL2 mentioned above plus words sourced from Chambers and Collins English dictionaries. This book is used to adjudicate at the World Scrabble Championship and all other major international competitions outside of North America.