Line clear

The object of Tetris is to last as long as possible before the screen fills up with tetrominoes. To do this, you must assemble the tetrominoes to form one or more rows of blocks that span the entire playing field, called a line clear. When you do so, the row will disappear, causing the ones above it to settle.

Types of line clears
Single

A single is the act of clearing one line at a time:

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Double

A double is when two lines are cleared at once:

Triple

A triple is three lines cleared simultaneously:

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Tetris

A tetris is four lines cleared simultaneously. Note that this can only be done with an "I" tetromino.

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Back-to-Back

Back-to-Back clears are any combination of two or more T-Spin Single, T-Spin Double, T-Spin triple, or Tetris, in the case of the recent game Tetris DS. In certain Tetris game modes, you can earn extra points or deal an additional line of garbage by clearing Back-to-Back.

For example, a player can clear a line with a T-Spin Single using the T tetromino, then clear four lines with an I tetromino, still earning him a Back-to-Back tetris.