From Hard Drop - Tetris Wiki
Double Triple Cannon 2, also known as DT Cannon 2, Double Triple 2 or DT2, is a variation on Double Triple Cannon. Rather than having a "large notch" (as with DT Cannon) that is cleared by a T-Spin Double followed by a T-Spin Triple, in DT Cannon 2, the T-Spin Double and T-Spin Triple are cleared by separate notches. However, similar to DT Cannon, DT Cannon 2 does have a notch for the T-Spin Double on the same side as its overhang.
Wall-Overhang Variation
This is the preferred way to set up DT Cannon 2.
Empty Field Setups
Situational Setups
Wall-Notches Variation
The wall-notches variation of DT Cannon 2 is actually one of the harder setups to master. It can only be made by transferring or trimming blocks, therefore it is only practical in mid-game.
Transferring:
As an Opener
Double Triple Cannon 2 can be used as an opener. There are better openers though.
DT2 C-Spin
This opener can be continued with a C-Spin. If one can stack the third and fourth bags like below, then they may have a chance of Perfect Clear after 14 lines.
If the third bag cannot let one stack the above, they can stack their third bag in one of the following two ways. The first is used when one gets an O early. The second is used then one gets an L early. There is no good way to place an early J piece.
C-Spin Perfect Clear setup from the first form:
C-Spin Perfect Clear setup from the second form:
Bad Parity
This section will explain why Double Triple Cannon 2 is barely ever used midgame. Thereby we will compare DT Cannon 2 with the standard DT Cannon. Imagine laying a checkerboard over the playfield. Parity describes the difference of filled cells on white and black squares. If a field has no holes / overhangs, then the difference of white and black squares on the surface is even twice as big as the overall difference (e.g. 2 more white cells than black cells in the field -> 7 white cells and 3 black cells on surface). Fields with a high difference of white and black cells on the surface are instable and should be avoided.
From the 7 different pieces, only the T Piece has an effect on Parity: it will shift the difference by 2. Thus it makes a difference whether you place the center of a T piece on a black square or white square. Line clears can also have an effect on Parity but that's not the case if you clear an even amount of lines directly above each other or if you clear lines at the very bottom. So, the T-Spin Double and T-Spin Triple will have no additional effect here besides being T placements.
In the following we will have a look at the empty cells in the DT Cannon builds and count how many black and white squares are filled by the subsequent T placements. While it's 4 white and 4 black squares for the standard DT Cannon, it's 6 white and 2 black squares (or 2 white and 6 black squares respectively) for DT Cannon 2. Basically in DT Cannon 2, the 2 T pieces are placed with the center on the same color (twice on black in the illustration below). The only other setup consisting of 2 T-Spins with this property is Fractal and that setup can be build more vertically.
Another reason why the standard DT Cannon is better is that the first T piece is placed more towards the bottom. Lines above the first T-Spin can be completed later - only the notch is needed beforehand.
DT Cannon 2 STSD
This setup looks identical to DT Cannon 2, with the only difference that one further cell is empty at the bottom (marked light grey in the first picture below). It results in an STSD after the first T-Spin Double is cleared. So, it is usually 3 T-Spin Doubles in total.
See also