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.
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
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). 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).
 Three Column Combo Setup
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).
 Four Column Combo Setup
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).
Various skimming methods can end up in combos.
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