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 
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Back-to-Back Recognition in Guideline Games
| Clear Type | Recognition |
|---|---|
| Single | No |
| Double | No |
| Triple | No |
| Tetris | Yes |
| T-Spin Single | Yes |
| T-Spin Double | Yes |
| T-Spin Triple | Yes |
BtB bonus |
Tetris |
Place J |
BtB Tetris |
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.
T-Spin → Tetris |
|
BtB Tetris |
Tetris → T-Spin |
Tetris |
BtB EZ TSS |
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.
No BtB bonus |
Tetris |
Double |
Non-BtB T-Spin Single |
Here, BtB is not recognized for the T-Spin line clear because the double line clear broke the BtB chain.
BtB bonus |
Double |
|