Back-to-Back

Back-to-Back (BtB) or Back-2-Back (B2B) clears are any combination of two or more "difficult" line clears without an "easy" line clear between them. The recent games Tetris Worlds and Tetris DS consider a 4-line clear ("tetris") or a T-spin line clear to be difficult.

Recognition
In the above diagrams, BtB is recognized for the second tetris line clear because the line clear right before was also a tetris. The J tetromino placement did not clear any lines and, therefore, does not break the BtB chain.

All types of "difficult" clears share the same state variable. 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.

Here, BtB is not recognized for the T-Spin line clear because the double line clear broke the BtB chain.

The above shows that BtB is recognized for the T-Spin line clear because the line clear right before was a tetris.

Reward
In certain Tetris game modes, you can earn extra points or deal an additional line of garbage by clearing Back-to-Back.