while each subsequent back-to-back Tetris is worth 1,200. In conjunction, players can be awarded combos that exist in certain games which reward multiple line clears in quick succession. The exact conditions for triggering combos, and the amount of importance assigned to them, vary from game to game. Nearly all Tetris games allow the player to press a button to increase the speed of the current piece's descent or cause the piece to drop and lock into place immediately, known as a "soft drop" and a "hard drop", respectively. While performing a soft drop, the player can also stop the piece's increased speed by releasing the button before the piece settles into place. Some games only allow either soft drop or hard drop; others have separate buttons for both. Many games award a number of points based on the height that the piece fell before locking, so using the hard drop generally awards more points. Infinite game question The question Would it be possible to play forever? was first considered in a thesis by John Brzustowski in 1992. The conclusion reached was that the game is statistically doomed to end. If a player receives a sufficiently large sequence of alternating S and Z Tetrominoes, the naïve gravity used by the standard game eventually forces the player to leave holes on the board. The holes will necessarily stack to the top and, ultimately, end the game. If the pieces are distributed randomly, this sequence will eventually occur. Thus, if a game with, for example, an ideal, uniform, uncorrelated random number generator is played long enough, any player will almost surely top out. Modern versions of Tetris released after 2001 use a bag-style randomizer that guarantees players will never receive more than four S and Z pieces in a row by shuffling tetrominoes of all types for each 7 pieces. This is one of the "Indispensable Rules" enforced by the Tetris Guideline that all officially licensed Tetris games must follow. Easy spin dispute "Easy spin", or "infinite spin", is a feature in some Tetris games where a tetromino stops falling for a moment after left or right movement or rotation, effectively allowing the player to suspend the piece while deciding where to place it. The mechanic was introduced in 1999's The Next Tetris, and drew criticism in reviews of 2001's Tetris Worlds. This feature has been implemented into the Tetris Company's official guideline.