Scrabble have been sold worldwide and roughly one-third of American and half of British homes have a Scrabble set. There are around 4,000 Scrabble clubs around the world. The game is played by two to four players on a square board with a 15×15 grid of cells (individually known as "squares"), each of which accommodates a single letter tile. In official club and tournament games, play is between two players or, occasionally, between two teams each of which collaborates on a single rack. The board is marked with "premium" squares, which multiply the number of points awarded: eight dark red "triple-word"
Tile tracking player who can plan and calculate the best move while taking into consideration the other player's possible responses—winning a close game by blocking an opponent’s big play or setting up a high-scoring play the opponent cannot block. Scrabble champion John Holgate notes that tile-tracking is particularly important in tight "bobbing" finishes when the bag is empty, and that many games are lost through "just not knowing what tiles your opponent has on her rack. You simply cannot calculate possible permutations if you are unsure about which letters are relevant." Tracking can also tell a player if the bag is ‘vowel-heavy’