Line clear

The object of Tetris is to last as long as possible before the screen fills up with Tetrominos. To do this, you must assemble the Tetrominos 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 certain Tetris game modes, you can earn extra points or deal an additional line of garbage by getting a Back-to-Back.

For Example: You could clear a line with a T-Spin Single using the T shaped block, then clear for lines with an I shaped block, earning you a Back-to-Back Tetris.