Tetris has been involved in many legal battles. In June 1984, Alexey Pajitnov created Tetris on an Elektronika 60 while working for the Soviet Academy of Sciences at their Computer Center in Moscow with Dmitry Pavlovsky, and Vadim Gerasimov ported it to the IBM PC. Gerasimov reports that Pajitnov chose the name "Tetris" as "a combination of 'tetramino' and 'tennis'." From there, the PC game exploded into popularity, and began spreading all around Moscow. The most recent version of this port is available on Gerasimov's web site.

The IBM PC version eventually made its way to Budapest, Hungary, where it was ported to various platforms and was "discovered" by a British software house named Andromeda. They attempted to contact Pajitnov to secure the rights for the PC version, but before the deal was firmly settled, they had already sold the rights to Spectrum HoloByte. After failing to settle the deal with Pajitnov, Andromeda attempted to license it from the Hungarian programmers instead.

Meanwhile, before any legal rights were settled, the Spectrum HoloByte IBM PC version of Tetris was released in the United States in 1986. The game's popularity was tremendous, and many players were instantly hooked; Computer Gaming World called the game "deceptively simple and insidiously addictive".

The details of the licensing issues were uncertain by this point, but in 1987 Andromeda managed to obtain copyright licensing for the IBM PC version and any other home computer system.

For Amiga and Atari ST two different versions by Spectrum HoloByte and Mirrorsoft became available. The Mirrorsoft version did not feature any background graphics while the Holobyte version had a background picture related to Russian themes for each level. Games were sold as budget titles due to the game's simplicity. Spectrum's Apple II package actually contained three diskettes with three different versions of the game, for the Apple II+ and Apple IIe on separate DOS 3.3 and ProDOS 5.25 in (133 mm) diskettes, and for the Apple IIgs on a 3.5 in (89 mm) diskette, none of which was copy-protected: the included documentation specifically charged the purchaser on his or her honor to not give away or copy the extra diskettes.

By 1988, the Soviet government began to market the rights to Tetris, after a promotional trip to the country by Gerald Hicks, the one time United States champion of the game, through an organization called Elektronorgtechnica, or "Elorg" for short. Pajitnov had granted his rights to the Soviet Government, via the Computer Center he worked at for ten years. By this time, Elorg had still seen no money from Andromeda, and yet Andromeda was licensing and sub-licensing rights that they themselves did not even have. In 1988, Sega released an arcade game version of Tetris. This Sega version won the Japanese Gamest Award for Game of the Year in 1989.

Nintendo

By 1989, half a dozen different companies claimed rights to create and distribute the Tetris software for home computers, game consoles, and handheld systems. Elorg, meanwhile, held that none of the companies were legally entitled to produce an arcade version, and signed those rights over to Atari Games, while it signed non-Japanese console and handheld rights over to Nintendo. Tetris was on show at the January 1988 Consumer Electronics Show in Las Vegas, where it was picked up by Dutch games publisher Henk Rogers, then based in Japan, which eventually led to an agreement brokered with Nintendo that saw Tetris bundled with every Game Boy.

Tengen (the console software division of Atari Games), regardless, applied for copyright for their Tetris game for the Nintendo Entertainment System, loosely based on the arcade version, and proceeded to market and distribute it under the name TETЯIS: The Soviet Mind Game (with faux Cyrillic typography incorporating the Cyrillic letter Ya), disregarding Nintendo's license from Elorg. Nintendo contacted Atari Games claiming they had stolen rights to Tetris, whereupon Atari Games sued, believing they had the rights. After only four weeks on the shelf, the courts ruled that Nintendo were the only company which had the rights to Tetris on home game systems, and Tengen's TETЯIS game was recalled, with an unknown number of copies sold. The lawsuits between Tengen and Nintendo over the Famicom/NES version carried on until 1993.

Nintendo released their version of Tetris for both the NES and the Game Boy (the Famicom and Game Boy versions were developed by Bullet-Proof Software, Inc., who held the Japanese license, despite Nintendo's overseas license to the game); The NES version sold 8 million copies worldwide. Some players considered Nintendo's NES version inferior because it lacked the side-by-side simultaneous play of Tengen's version, but Nintendo's Game Boy Tetris became arguably the most well-known version of Tetris, selling over 33 million copies.

Sega

Sega also released a Tetris game for the Mega Drive; however, the ensuing blitz of litigation ensured that it was hastily withdrawn. However, it is estimated that between 3 to 8 copies of Tetris were produced for the Japanese Sega Mega Drive. Due to its extremely high rarity, and because Tetris is a very important and very popular game in video game history, Tetris Mega Drive is a sought after item for video game collectors. It is one of the "holy grails" of video game collecting..

The Tetris Company

In 1996, the rights to the game reverted from the Russian state to Pajitnov himself, who previously had made very little money from the game. In 1996, The Tetris Company was founded, claiming to hold copyright registrations for Tetris products in the U.S. and taking out trademark registrations for Tetris in almost every country in the world. They have licensed the brand to a number of companies, and the U.S. Court of International Trade and the U.S. Customs have at times issued seizure orders to preclude unlicensed Tetris-like games from being imported into the U.S., though bulletins circulated by the U.S. Copyright Office state that copyright does not apply to the rules of a game.

In mid-2006, and in late 1997, TTC's legal counsel sent cease and desist letters to Web sites on the basis of Tetris-type games infringing the "Tetris" trademark, trade dress, and/or "look and feel" copyright. Around 2009, TTC and Tetris Holding LLC brought legal action against BioSocia, Inc., on the grounds that BioSocia's "Blockles" game infringed on proprietary rights that were held by TTC and Tetris Holding LLC. On September 10, 2009, the legal case against BioSocia was resolved, with BioSocia agreeing to discontinue making the "Blockles" game available to the public. In May 2010, TTC's legal counsel sent cease and desist letters to Google insisting that 35 Tetris clones be removed from the Android Market. A US District Court judge ruled in June 2012 that the Tetris clone "Mino" from Xio Interactive infringed on the Tetris Company's copyrights by replicating such elements as the playfield dimensions and the shapes of the blocks.

Tetris has been subject to many changes throughout releases since the 1980s. Newer Tetris games have focused on a trend of pace rather than endurance. Older releases such as Game Boy or NES Tetris offer records according to points. Since the meter for points is restricted to a specific number of digits, these games' records can be "maxed out" by an experienced player. The next big Game Boy release after Tetris, Tetris DX, in marathon mode—comparable to mode A in previous releases—allowed an additional digit for the point meter. Even so, players still maxed it to 9,999,999 points after hours of play. For The New Tetris, world record competitors have spent over 12 hours playing the same game. In Tetris DX and The New Tetris, the new modes sprint and ultra were added. These modes require the player to act under a timer, either to gain the most lines or points in that time. Releases like Tetris Worlds did away completely with point records. This particular game kept records by how fast a certain number of lines could be cleared depending on the level. Critics of Tetris Worlds said it was flawed by virtue of the ability of a piece to hover over the bottom for as long as a player needs.

There are many different modes of play added in recent years. Modes appearing in more than one major release include: classic marathon (game A), sprint (otherwise game B or 40 lines), ultra, square, and cascade.

The field dimension of Tetris is perhaps the least deviated among releases: almost always 10 cells wide by 20 high. Some releases on handheld platforms with small screens have smaller fields; for example, the Tetris Jr. keychain game has 8 by 12, and Tetris for Game Boy has 10 by 18.

Traditionally, blocks spawn within the four most central columns and the two highest rows. The I Tetrimino occupies columns 4, 5, 6 and 7, the O Tetrimino occupies columns 5 and 6, and the remaining 5 Tetriminos occupy columns 4, 5 and 6 (or in some, especially older, versions, 5, 6 and 7). In some more recent games, pieces spawn above the visible playfield.

In traditional games, a level-up would occur once every ten lines are cleared. After a level-up, the blocks fall slightly faster, and typically more points are given. In some newer games such as Tetris Worlds, the number of lines required vary upon each new level. For example, NES Tetris operates at 60 frames per second. At level 0, a piece falls one step every 48 frames, and at level 19, a piece falls one step every 2 frames. Level increments either terminate at a certain point (Game Boy Tetris tops off at level 20) or increase forever yet not in speed after a certain point. NES Tetris will level up in speed until level 29 (due to limitations of the game's engine, pieces are not capable of dropping faster than this), but tool-assisted emulation will show that the level indicator increases indefinitely—eventually leading to a glitch where the meter displays non-numeric characters. Modern games such as Tetris the Grand Master or Tetris Worlds, at their highest levels, opt to drop a piece more than one row per frame. Pieces will appear to reach the bottom as soon as they spawn. As a result, these games have a delay that lets the player slide the piece on the bottom for a moment to help deal with an otherwise unplayable fall speed. In some games, the hover time is regenerated after a piece is moved or rotated.

Soft drops were first implemented in Nintendo releases of Tetris so that pieces would be able to drop faster while not locking as to slide into gaps. The other option is hard dropping, which originated in early PC games such as Microsoft Tetris, a game developed by Dave Edson and bundled with Microsoft Entertainment Pack. With hard dropping, a piece falls and locks in one frame. Newer Tetris games feature both options. Some games have their locking roles reversed, with soft dropping making the pieces drop faster and locking down, and hard dropping making the pieces drop instantly but not lock.

Single direction rotation is an older restriction that has since been ruled out in nearly every new official release by the favor of separate buttons for clockwise and one for counterclockwise rotation. In traditional games, the unsymmetrical vertical orientation I-, Z-, and S-pieces will fill the same columns for each clockwise and counter clockwise rotation. Some games vary this by allowing two possible column orientations: one for counter clockwise and one for clockwise rotations. Double rotation—only seen in progressive clones such as Quadra and DTET—rotates the piece 180°.

One of the features most appreciated by skilled players is wall kick, or the ability of rotating the pieces even if these touch the left or right walls. In the NES version, for example, if a Z piece is "vertically" aligned and falling touching the left wall, the player cannot rotate the piece, giving the impression that the rotate buttons are locked. In this situation, the player has to move the piece one position to the right before rotating it, losing precious time. Proper implementations of wall kick first appeared in the arcade version of Tetris by Atari Games.

Piece preview allows a look at the next piece to enter the field. This feature has been implemented since the earliest games, though in those early games, having the preview turned on made the score increase more slowly.

Newest features

Newer versions of Tetris add different scoring goals not present in traditional Tetris. As achieving these goals while not topping out becomes more difficult, these games usually add a few features to help the player.

The New Tetris and The Next Tetris are the first official Tetris games to feature multiple piece previews, showing 3 in advance. Tetris Worlds for PCs and game consoles add 5 more, while the GBA version retains the 3 piece preview. Tetris DS uses the 6-piece preview. The New Tetris also introduced the "ghost piece", an obscuration in the shape of the current piece over where that piece would drop. The feature reduces mistakes, especially for beginners and high-speed players.

Hold piece is an optional ability to reserve a piece for later use, allowing a player to either avoid undesirable pieces or save desirable ones. Some clones featured it as a powerup that the player could earn and use once. A hold piece available to the player at all times was first featured in The New Tetris. Most games that have hold piece activate it when the player presses a dedicated button, often a shoulder button; other games activate it when both rotate buttons are pressed simultaneously. When hold piece is activated, it causes the falling piece to move to the top and trade places with the hold piece. However, the feature cannot be activated twice in a row; it is disabled until the piece released from hold locks in the well.

Initial rotation and Initial hold are features that make the game accept rotation/hold button inputs while the next piece is still in the preview area. With initial rotation, when the player holds down the rotation button after the previous piece has locked down but before the next piece comes into the well, the next piece will come into the well in an already rotated state. Initial hold works similarly, as the piece will be already swapped with the hold piece when it enters the well. Initial rotation and Initial hold first appeared in the Tetris: The Grand Master series.

Tetris DS features wireless on-line play through the Nintendo Wi-Fi Connection system. This new version also takes advantage of the touch screen in the added "Touch Mode," which has no time limit. Instead, every block is already placed in a tall tower, and the player uses the stylus from the Nintendo DS to shift blocks left and right and, in earlier towers, rotate blocks. The goal is to clear enough lines so that a cage of balloons reaches the ground (this mode is themed on the NES video game Balloon Fight, hence the cage of balloons).

Tetris DS also introduces the Metroid-themed "Catch Mode". In this mode, the pieces fall downward from the top screen to the touch screen, but the stack is moved and rotated instead. As the falling pieces bump against the stack, they get clustered into it. To clear blocks, there must be a solid area of the stack that is 4×4 or larger. When this happens, the blocks glow and the music changes. After ten seconds or upon pressing the X button, these blocks disappear and shoot a laser beam in a plus-shape, the horizontal part equal to the number of rows cleared and the vertical equal to the columns. This laser beam will move and rotate with the stack and destroy falling blocks and Metroid enemies in its path until it disappears a moment later. The parts of the stack not hit by the laser beam will be pulled in towards the center of the stack after the laser beam dies. If a piece reaches the bottom of the touch screen, the stack hits a falling block while rotating, or the stack hits a Metroid, the stack loses Energy. The player loses if the stack runs out of Energy or if the stack becomes so large that it can no longer fit on the touch screen.

Tetris Mania by EA Games brings back the Fusion and Sticky modes from Tetris Worlds. In Fusion, "atom" blocks must be activated, the number of those needing to be activated increases per level. Activated atoms wills also activate other atoms that they touch, and are generated two for every seven Tetriminos. Gravity will not be activated until a line is cleared containing an atom of fusion block. In Sticky, based on The Next Tetris, you need to clear the bottom row of starting tiles. In each level, there are more starting tiles that are harder to clear. The pieces in this game are made up of different colored minos that "stick" to those of the same color. Gravity is always a factor.

The Tetris arcade game by Atari Games offered different "puzzles" for selected rounds. The first three rounds are played normally, with no obstacles. At the start of round 4, eight bricks are placed vertically along each side of the well. Round 5 begins with ten bricks scattered throughout the bottom five rows. Round 6 begins with 20 bricks arranged in a pyramid. In rounds 7 through 9, the well starts out empty but single bricks will appear at random on top of your puzzle each time a piece lands that does not clear any lines, potentially thwarting any planning one may have done. In rounds 10 through 12, incomplete "garbage" lines will randomly pop up underneath your puzzle, pushing the puzzle upward, when a piece lands without clearing any lines. Rounds 13 through 15 begin with more blocks arranged in predetermined patterns, and the cycle continues throughout the remaining rounds in the game in groups of three.

Several Tetris variants exist. Some feature alternate rules and pieces, and others have completely different gameplay.

A popular variant called "The Grand Master" eventually becomes so fast players have to use every second of time optimally, and it even has a mode dubbed "Invisible Tetris", where the blocks are only shown when falling – then finally revealed when the game is over.

Because of its popularity and the relatively simple code required to produce the game, a game with nearly the same rules as Tetris is often used as a hello world project for programmers coding for a new system or programming language. This has resulted in the availability of a large number of ports for different platforms. For instance, µTorrent and GNU Emacs contain similar shape-stacking games as easter eggs.

Players lose a typical game of Tetris when they can no longer keep up with the increasing speed, and the Tetriminos stack up to the top of the playing field. This is commonly referred to as topping out.

Possibility of indefinite gameplay

The question Would it be possible to play forever? was first encountered in a thesis by John Brzustowski in 1988. The conclusion reached was that the game is inevitably doomed to end. The reason has to do with the S and Z Tetriminos. If a player receives a large sequence of alternating S and Z Tetriminos, the naïve gravity used by the standard game eventually forces the player to leave a hole in a corner.

Supposing that the player then receives a large sequence of alternating S and Z Tetriminos, they will eventually be forced to leave holes throughout the board. Back and forth, 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 an ideal, uniform, uncorrelated random number generator is played long enough, any player will top out.

In practice, this does not occur in most Tetris variants. Some variants allow the player to choose to play with only S and Z Tetriminos, and a good player may survive well over 150 consecutive Tetriminos this way. On an implementation with an ideal uniform randomizer, the probability at any given time of the next 150 Tetriminos being only S and Z is one in (2/7) (approximately 2×10). Most implementations use a pseudorandom number generator to generate the sequence of Tetriminos, and such an S–Z sequence is almost certainly not contained in the sequence produced by the 32-bit linear congruential generator in many implementations (which has roughly 4.2×10 states). The "evil" algorithm in Bastet (an unofficial variant) often starts a game with a series of more than seven Z pieces.

Recent versions of Tetris such as Tetris Worlds allow the player to repeatedly rotate a block once it hits the bottom of the playfield, without it locking into place (see Easy spin dispute, above). This permits a player to play for an infinite amount of time, though not necessarily to land an infinite number of blocks.

In computer science, it is common to analyze the computational complexity of problems, including real life problems and games. It was proven that for the offline version of Tetris (in which all the pieces are known in advance) the following objectives are NP-complete:

• Maximizing the number of rows cleared while playing the given piece sequence.
• Maximizing the number of pieces placed before a loss occurs.
• Maximizing the number of simultaneous clearing of four rows.
• Minimizing the height of the highest filled grid square over the course of the sequence.

Also, it is not possible to find a polynomial time approximation algorithm for the first 2 problems and it is hard to approximate the last problem within 2 − ε for every ε > 0.

To prove NP-completeness, it was shown that there is a polynomial reduction between the 3-partition problem, which is also NP-Complete, and the Tetris problem.

• Music A in version 1.1 of the Game Boy edition of Tetris has become very widely known, to the point that Level 20 in Tetris DS is based on this Game Boy version of Tetris and uses that theme. It is an instrumental arrangement of a Russian folk tune called "Korobeiniki" (the most widely used of several Romanized spellings), which has been covered by UK dance band Doctor Spin, US alternative rock band Ozma, Tokyo Ska Paradise Orchestra, Basshunter the Swedish Eurodance DJ, Doctor P,and also the German techno group Scooter on their 2007 album Jumping All Over the World. It was also sampled in "21 Concepts" by MC Lars. Music A and B are also remixed and arranged for Super Smash Bros. Brawl, and can be selected for the stage "Luigi's Mansion", as well as being used in custom stages. The song has also been remixed for two dance games, under the name "Pumptris Quattro" in Pump It Up NX2 and "Happy-hopper" in Dance Maniax 2nd Mix. Ronan Murray has recorded an arrangement of the tune for pipe organ. A remix was also made by the Hip-hop group Decav5.
• Music B in the Game Boy version is likely by Hirokazu Tanaka.
• Music C in the Game Boy version is an arrangement of Johann Sebastian Bach's French Suite No. 3 In B Minor, BWV 814, IV. Menuett – Trio.
• Music 1 in the NES version is "Dance of the Sugar Plum Fairy", a tune noted to be scene 14c of act two of The Nutcracker, composed by Tchaikovsky.
• One song in the BPS and Tengen versions is the "Kalinka", a famous Russian song written by Ivan Petrovich Larionov.
• A 6.06 minute concert version has been composed by Nikolai Aleeksevich Nekraov

According to research from Dr. Richard Haier, et al. prolonged Tetris activity can also lead to more efficient brain activity during play. When first playing Tetris, brain function and activity increases, along with greater cerebral energy consumption, measured by glucose metabolic rate. As Tetris players become more proficient, their brains show a reduced consumption of glucose, indicating more efficient brain activity for this task. Even moderate playing of Tetris (half-an-hour a day for three months) boosts general cognitive functions such as "critical thinking, reasoning, language and processing" and increases cerebral cortex thickness.

In January 2009, an Oxford University research group headed by Dr. Emily Holmes reported in PLoS ONE that for healthy volunteers, playing Tetris soon after viewing traumatic material in the laboratory reduced the number of flashbacks to those scenes in the following week. They believe that the computer game may disrupt the memories that are retained of the sights and sounds witnessed at the time, and which are later re-experienced through involuntary, distressing flashbacks of that moment. The group hopes to develop this approach further as a potential intervention to reduce the flashbacks experienced in posttraumatic stress disorder, but emphasized that these are only preliminary results.

The game has been noted to cause the brain to involuntarily picture tetris combinations even when the player is not playing (the Tetris effect), although this can occur with any computer game or situation showcasing repeated images or scenarios, such as a jigsaw puzzle.

Reception

The IBM version of the game was reviewed in 1988 in Dragon No. 135 by Hartley, Patricia, and Kirk Lesser in "The Role of Computers" column. The reviewers gave the game 4.5 out of 5 stars. The Lessers later reviewed Spectrum HoloByte's Macintosh version of Tetris in 1989 in Dragon No. 141, giving that version 5 out of 5 stars. In 1993, the Spectrum version of the game was voted number 49 in the Your Sinclair Official Top 100 Games of All Time. In 1996, Tetris Pro was ranked the 38th best game of all time by Amiga Power.

In 2007, video game website GameFAQs hosted its sixth annual "Character Battle", in which the users nominate their favorite video game characters for a popularity contest in which characters participate. The L-shaped Tetris piece (or "L-Block" as it was called) entered the contest as a joke character, but on November 4, 2007, it won the contest. On June 6, 2009, Google honored Tetris' 25-year anniversary by changing its logotype to a version drawn with Tetris blocks – the "l" letter being the long Tetris block lowering into its place, seen here. In 2009, Game Informer put Tetris 3rd on their list of "The Top 200 Games of All Time", saying that "If a game could be considered ageless, it's Tetris".

In research

Tetris has been the subject of academic research. Vladimir Pokhilko was the first clinical psychologist to conduct experiments using Tetris. Subsequently, it has has been used for research in several fields including theory of computation, algorithmic theory and cognitive psychology.

During the game of Tetris, blocks appear to adsorb onto the lower surface of the window. This has led scientists to use tetrominoes "as a proxy for molecules with a complex shape" to model their "adsorption on a flat surface" to study the thermodynamics of nanoparticles.

• List of Tetris variants
• Game Over — 1993 book covering Nintendo history, including interviews with Alexey Pajitnov and others regarding Tetris licensing.
• Lumines
• Brain Wall and Blokken — game shows based on Tetris
• Ecstasy of Order: The Tetris Masters — 2011 documentary about the 2010 Tetris Championships featuring interviews with Pajitnov and Dr. Richard Haier.