From Hard Drop - Tetris Wiki
The Tetris Guideline is the current specification that The Tetris Company enforces for making all new (2001 and later) Tetris game products alike in form. A mode labeled "standard" or just plain "Tetris" will adhere to the Guideline more closely than other modes.
As of now, the Guideline is conjectured to encompass the following rules, learned through observation of the behavior of authentic Tetris games:
 Indispensable rules
- Playfield is 10 cells wide and at least 22 cells tall, where rows above 20 are hidden or obstructed by the field frame
- Tetromino colors
- Cyan I
- Yellow O
- Purple T
- Green S
- Red Z
- Blue J
- Orange L
- Tetromino start locations
- The I and O spawn in the middle columns
- The rest spawn in the left-middle columns
- The tetrominoes spawn horizontally with J, L and T spawning flat-side first.
- Super Rotation System/Standard Rotation System (SRS) specifies tetromino rotation
- Standard mappings for console and handheld gamepads:
- Standard mappings different from console/handheld gamepads for computer keyboards
- So-called Random Generator (also called "random bag" or "7 system")
- "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. Games on platforms with fewer than eight usable buttons (such as the version on iPod) may skip this feature. The combination of hold piece and Random Generator would appear to allow the player to play forever.
- Game must have ghost piece function.
- Terms used in the user manual: "Tetriminos", as opposed to "tetrominoes", "tetrads" or "pieces"; letter names, as opposed to "square", "stick", etc.
- Designated soft drop speed. Details vary between guideline versions.
- Player may only level up by clearing lines or performing T-Spins. Required lines depends on the game.
- The game must use a variant of Roger Dean's Tetris logo, although this was true from around 2000 - before the guidelines emerged.
- Game must include a song called Korobeiniki, which is the Tetris theme song. (Guideline 2005~)
- The player tops out when a piece is spawned overlapping at least one block, or a piece locks completely above the visible portion of the playfield.
 Recommended but non-mandatory
- Display of next-coming tetrominoes. Most games show at least three, though there are no hard rules.
- Recognition and rewarding of T-spin moves. Conditions vary between guideline versions.
- 2005: 3-corner T
- 2006: 3-corner T no kick
- Rewarding of Back to Back chains. Recognition method depends on the game.
- Game should include the songs Katjusha, or Kalinka.
- DAS no faster than Tetris Zone. (However, some Japanese games blend the Guideline with the Sega tradition of 1G DAS: Tetris New Century, Kiwamemichi, and the TGM collection.)
The extent to which the Guideline specifies the speed curve, the scoring system, and other aspects not listed on this page, is not yet known to the public.
Although Guideline-compliant games share many traits, they also have differences in many aspects as well. There are a few instances where a game will break a trait which is shared by all other games thought to be compliant. Examples of this include the lack of the hold function and the T-spin's ability to start and continue Back-to-Back chains in iPod Tetris, and the inverted rotation button layout of TGM3 and TGM ACE (or Kiwamemichi, depending on interpretation). No explanations have been given for the reasons of these games' deviations.
Certain games, such as Tetris Online (Japan) and the handheld electronic games by Radica Games defy the guidelines despite having "Authentic Tetris Game" labels. In other words, tetris games don't have to follow the Guideline to be official.
 Guideline Versions
Henk Rogers has been quoted as revising the guideline annually. This may only adhere to 2005 and later, as so far only guideline 2002, 2005 and 2006 have been known to be in existence, and guideline 2002 games have been built from scratch and released as late as 2005.
So far, the first confirmed guideline 2005 games (TGM ACE) has been released in December 2005, while the first confirmed guideline 2006 game (Tetris: New Century, although iPod Tetris released earlier behaves similarly) was released in September 2006. Therefore, it can be conjectured that the schedule of a new guideline version is released somewhere between the middle to the latter half of the year.
While many games have no publicly visible indication of the guideline version by the developer or publisher, some games have had their exact guideline versions made clear by them. They are listed in the Guideline compliant game differences page.
 See also
- Guideline compliant game differences
- Tetris Worlds, Tetris Deluxe, and Tetris Zone, considered by many to be the "reference" implementations of the Guideline at the respective times of the games' releases.
- Tetris DS, another popular game that is compliant with the Guideline