Scrabble of points for each lettered tile is based on the letter's frequency in standard English; commonly used letters such as vowels are worth one point, while less common letters score higher, with Q and Z each worth 10 points. The game also has two blank tiles that are unmarked and carry no point value. The blank tiles can be used as substitutes for any letter; once laid on the board, however, the choice is fixed. Other language sets use different letter set distributions with different point values. Tiles are usually made of wood or plastic and are square and thick,
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’