The wink on top may be played, though, and sophisticated play involves shots manipulating large piles of winks. The game ends in one of two ways: either all the winks of one colour are potted (a "pot-out"), or play continues up to a specified time limit (usually 25 minutes), after which each colour has a further five turns. Then a scoring system is used to rank the players, based on the numbers of potted and unsquopped winks of each colour. There are two national associations, the English Tiddlywinks Association (ETwA) and the North American Tiddlywinks Association (NATwA) (the Scottish Tiddlywinks Association having disbanded in the late 1990s). These organisations are responsible for conducting tournaments and maintaining the rules of the game (which actually differ only slightly between the two organisations; the NATwA rules are based on the ETwA rules). International competition is overseen by the International Federation of Tiddlywinks Associations (IFTwA), though in practice it is rarely called upon to intervene. Although tiddlywinks nowadays is a singles or pairs game, competition in the 1950s until the 2000s centred on team competition, with teams consisting of several (two to four) pairs. There were a number of university teams, and international matches were also played. More recently, singles and pairs tournaments have come to be the focus of competitive tiddlywinks, with only a few team matches being played each year. The four most prestigious tournaments are the National Singles and National Pairs tournaments held in England and the United States. The World Singles and World Pairs championships operate on a challenge basis; anyone winning a national tournament (or being the highest-placed home player behind a foreign winner) is entitled to challenge the current champion. There are several other less prestigious tournaments in England and the United States throughout the year, often with a format designed to encourage inexperienced players. The results of tournaments and world championship matches are used to calculate Tiddlywinks Ratings, which give a ranking of players. The important appeal of the game for many players is the required combination of manual dexterity and strategic thought as well as tactics. Tiddlywinkers often claim that the game combines physical skill (such as in snooker or golf) with the strategy of chess. Tiddlywinks is unique in the combination of skill and strategy it requires. Strategy in tiddlywinks is often rather deep, since winks can be captured.