Tetris Guideline

The Tetris guideline is the current specification that The Tetris Company enforces for making all new (2001 and later) Tetris games alike in form. A mode labeled "standard" or just plain "Tetris" will adhere to the Guideline more closely than other modes.

Current rules, learned through observation of the behavior of authentic Tetris games, include:
 * Playfield at least 10 cells wide and 22 cells tall, where the top two may be hidden
 * Tetromino colors
 * Cyan I
 * Yellow O
 * Purple T
 * Green S
 * Red Z
 * Blue J
 * Orange L
 * At least three next pieces visible
 * "Random Generator" (also called "random bag" or "7 system")
 * Game is divided into rounds. In each round, all seven tetrominoes are generated in a random permutation. (There are 7!, or 5,040, distinct permutations of seven elements.) Then this permutation is inserted into the next queue. Unlike the Memoryless generator in classic games, the Random Generator can only produce a maximum of 12 tetrominoes between one I and the next I, and a run of S and Z tetrominoes is limited to a maxumim of 4. Despite the generic sounding name, presumed employees of BPS are known to treat the term "Random Generator" as a unique name, referring only to this particular algorithm.
 * Tetromino spawn locations
 * The I and O spawn in the middle columns
 * The rest spawn in the left-middle columns
 * The tetrominoes spawn horizontally and with their flat side pointed down.
 * Super Rotation System (SRS) specifies tetromino rotation and lockdown behavior
 * "Hold piece": The player can press a button to send the falling tetromino to the hold box, and any tetromino that had been in the hold box moves to the top of the screen and begins falling. Hold cannot be used again until after the piece locks down.

The combination of hold piece and Random Generator would appear to allow the player to play forever.