20G

The term 20g refers to the block moving 20 units per frame, a.k.a instant drop. The behavior of this mode closely resembles if one were to hold up in Tetris the Grand Master 2.

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The block cannot move across other minos if the playing field is uneven. |

       Playing field

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         T moved right

Because of this, the best possible stack in 20g is a pyramid;

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Allowing the player to put his tetromino anywhere on the playing field.

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Death mode

A mode made famous by Youtube, featuring instantly dropping Tetrominoes. The difference between death mode and 20g is un-noticeable until later stages in the game in which ARE starts to decrease and lock delay is very low. Arika currently hosts a video showing a GM grade in Death mode here.