From Hard Drop - Tetris Wiki
Combos can be a powerful tool in multiplayer games, and also can be useful in singleplayer games which reward combo chains well. For these reasons, some players will set their field up for a long combo chain at the very start. Also, downstacking and various skimming methods can end up in combos.
There are many different ways to setup for a long combo chain at the start of a round, but they all are variations of 3 known combo setups: 2 column, 3 column, and 4 column. The difficulty and maximum combo length increases with each additional column gap.
 Two Column Combo Setup ("2-Wide")
By far the easiest combo setup is done by leaving two columns open. This method of comboing is popular in TGM modes because of its high potential for double line clears (in TGM single line clears with consecutive pieces do not increase a combo). It is also a fairly common in Cultris and Cultris II because of the extra combo time gained by multi-line clears and the hard nature of these games (1 preview, no hold, mean randomizer).
While fairly easy to maintain, in games with garbage blocking the maximum combo chain for a 2 column setup is only 10 (in a 22h field). It is recommended to build a few rows of 4-wide and 3-wide on top of it in order to drop L, J and I pieces horizontally which may increase the combo chain length.
 Three Column Combo Setup ("3-Wide")
The 3 Column Combo Setup is a balance between difficulty and maximum combo chain length. This method of comboing is popular in guideline multiplayer games because it allows for a large amount of single line clears and is less risky to perform than the 4 column combo setup.
In games with garbage blocking the maximum combo chain for a 3 column setup increases to 16 (in a 22h field). It is recommended to build one or two rows of 4-wide on top of it in order to drop I pieces horizontally.
 Four Column Combo Setup ("4-Wide")
The 4 Column Combo Setup is the most difficult to maintain and is the riskiest to go for, but also allows the longest chains and is also the most damaging in multiplayer. The key to maintaining a combo chain with this setup is by clearing lines in a manner so that you are left with a 1 hole gap for your next line clear, or 2 to 3 holes that are adjacent to eachother. Breaking this rule will cause a S, Z, or O piece to end your combo chain.
This method of comboing is used by experienced players who plan ahead several pieces accordingly. Nearly every line clear in this method is a single. In games with garbage blocking the maximum combo chain for a 4 column setup is 21 (in a 22h field). A 4-wide setup will only work properly with Hold feature available. Even under Guideline rules, the player probably needs to utilize SRS wall kicks from time to time, which may slow down the execution (especially with slow softdrop speed).
 Center Techniques
Some players also leave the gap in the middle. This is much harder to stack, but has a significant defensive advantage. Usually, a topout only appears when a new piece spawns in collision with the stack. Leaving the 4 center columns empty will prevent this. This technique is often called center 4-wide (opposed to the very common sidestacked 4-wide). A center 3-wide is also possible, but in this case a new dealt I piece leads to a top-out (unless Initial Rotation System is available).
 4-Wide with 3 Residua
Below we assume guideline rules, that means 7-bag randomizer, Hold feature, and SRS kicks. Let's assume that the player has stacked very high in the left 6 columns and has filled exactly 3 cells in the right 4 columns.
Let's further assume that each further piece placement results in a Single line clear. In this case, there will be exactly 3 filled cells in the right 4 columns for the next couple of moves. The following table lists all configurations for the right 4 columns (besides mirroring) which won't break the combo and shows all possible ways to continue the combo. Note that the I piece can always be dropped horizontally - with it clearing a line and not changing the configuration - so it is left out of consideration.
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