A pseudorandom sequence of tetrominoes (sometimes called "tetrads" in older versions) - shapes composed of four square blocks each - fall down the playing field. The object of the game is to manipulate these tetrominoes, by moving each one sideways and rotating it by 90 degree units, with the aim of creating a horizontal line of blocks without gaps. When such a line is created, it disappears, and the blocks above (if any) fall. As the game progresses, the tetrominoes fall faster, and the game ends when the player "tops out", that is, when the stack of tetrominoes reaches the top of the playing field and no new tetrominoes are able to enter. (The exact definition of a top-out varies from version to version.)

      Players refer to the seven one-sided tetrominoes in Tetris as I, J, L, O, S, T, and Z. All are capable of single and double clears. I, J, and L are able to clear triples. Only the I tetromino has the capacity to clear four lines simultaneously, and this is referred to as a "Tetris."^[2] (This may vary depending on the rotation and compensation rules of each specific Tetris implementation. For instance, in the Tetris Worlds type rules (see below) used in many recent implementations, certain rare situations allow T, S and Z to 'snap' into tight spots, clearing triples.)

 

Gravity

      Traditional versions of Tetris move the stacks of blocks down by a distance exactly equal to the height of the cleared rows below them. Unlike Newtonian gravity, blocks may be left floating above gaps. This behavior is known as "naïve gravity." Some variants implement a different algorithm that uses a flood fill to segment the playfield into connected regions and then makes each region fall individually, in parallel, until it touches the region at the bottom of the playfield. This opens up additional "chain-reaction" tactics involving blocks cascading to fill additional lines, which may be awarded as more valuable clears.

Original algorithm.

Algorithm with chain reactions.

     The scoring formula for the majority of implementations of Tetris is built on the idea that more difficult line clears should be awarded more points. Nintendo's implementations on the NES, Game Boy, and SNES use what is probably the most widely recognized system.^[3]

     On most implementations, players may press a button to accelerate the current piece's descent, rather than waiting for it to fall. When a player locks a piece in this way, many such versions award a number of points based on the height the piece fell before locking. If a piece is manually dropped x lines and locked before the button is released, these versions will typically award either x points, or (level + 1)*x points. If a piece is not accelerated at all the player will gain no points for that piece unless a line is made.

Easy spin dispute

      Although not the first Tetris game to feature "easy spin" (see The Next Tetris), or also called "infinite spin" by critics, Tetris Worlds was the first game to fall under major criticisms for it. Easy spin refers to the ability of a tetromino's lockdown time to regenerate after left or right movement or rotation, and this has been implemented into The Tetris Company's official guideline. This new type of play differs from traditional Tetris because it takes away the pressure of higher level speed. Some reviewers even went so far to say that this mechanism broke the game. The goal in Tetris Worlds however, has to do with completing a certain number of lines as fast as possible, so technically the ability to hold off a piece's placement will not make achieving that goal any faster. Later, critics would receive "easy spin" more openly, saying "though the infinite spin issue honestly really affects only a few of the single-player game play modes in Tetris DS, because any competitive mode requires you to lay down pieces as quickly as humanly possible." [2] In response to the issue, Henk Rogers stated in an interview [3]:

So the problem is you get part way through the game, make one small mistake, 'Aw shit, I blew it,' and restart. I think that's an annoying way to play the game. So we decided it's better to give them a way to recover from that small mistake, but you're losing time. So if you sat there and rotated for, I don't know, five seconds, you've just taken five seconds out of the game that you needed to score so many points. So you won't find in the top games any gratuitous spinning going on, it just doesn't happen. It helps the beginning player who's trying to figure out what to do. It's a useless feature (for competitive play); it only helps if you're taking the time to think. The better players don't take that much time to think, that's the difference.

      Despite this, some recent games are not implemented in the way he meant for it to work. Modes lacking a timer such as marathon in Tetris Deluxe or also marathon and line mode in Tetris DS make problems for his statement. The claim that "you've just taken five seconds out of the game that you needed to score so many points" isn't true. Losing time by gratuitously spinning a piece will not hurt the player in modes that do not reward better times with better scores.