Gameplay of Tetris
The action of Tetris happens within a machine called a tetrion.
The largest part of the tetrion is a playfield consisting of a 10 by 20 space grid. It has other parts explained below.
Randomly selected tetrominoes, or shapes consisting of four square blocks, fall from the top of the playfield one at a time. Each tetromino enters the playfield with a given orientation and color depending on its shape. The piece preview is a part of the tetrion that shows the next pieces that will enter the playfield.
The player can rotate the falling tetromino 90 degrees at a time within the plane of the playfield by pressing the counterclockwise or clockwise rotation button, provided that there is space for it to rotate. Some versions of the game nudge the tetromino away from the wall or other blocks in order to make room.
The player can shift the falling tetromino sideways one space at a time by pressing the left or right arrow or holding it for quicker movement, provided that there is space for it to move. It cannot be shifted through the walls, or sides of the playfield, or through other blocks.
At the top left of the tetrion is an area called the hold box in which a tetromino can be stored for later use. At any time while a tetromino is falling, the player can move it to the hold box, and any tetromino that was in the hold box move to the top of the playfield. A tetromino moved out of hold cannot be moved back into hold; it must be locked (see below).
Each tetromino moves downward slowly by itself. There is generally a method for the player to "drop" the tetromino, or make it move downward more quickly. Once the tetromino lands on the floor or other blocks, there is a short delay in which the player can move the piece before it locks into place. After the tetromino locks, it cannot be moved further.
If tetrominoes lock into a position that fills in all spaces in one or more rows of the playfield with blocks, this is a line clear. The blocks in those rows are removed, and the blocks above them move down by as many rows as were removed.
If the playfield has not filled up with blocks, the next piece enters the playfield.