The object of Tetris is to last as long as possible before the screen fills up with tetrominoes. To do this, you must assemble the tetrominoes to form one or more rows of blocks that span the entire playing field, called a line clear. When you do so, the row will disappear, causing the ones above it to settle.
Types of line clears 
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A single is the act of clearing one line at a time:
A double is when two lines are cleared at once:
A triple is three lines cleared simultaneously. Most games allow only I, L, and J tetrominoes to complete a triple, but newer games with SRS allow S, Z, and T tetrominoes to twist into seemingly impossible positions.
A tetris is four lines cleared simultaneously. In most games, this can only be done with an "I" tetromino.
Hurdle or SplitEdit
In Tetris DS mission mode, the act of clearing two or three lines separated by one or more rows with gaps is called a hurdle. In the 1998 Sega Tetris, this move was called a split and it launched a special attack in VS mode. Any tetromino that can complete a triple can complete a hurdle:
Back-to-BackEdit
Back-to-Back 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. In certain Tetris game modes, you can earn extra points or deal an additional line of garbage by clearing Back-to-Back.
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.
A combo refers to making a line clear with one piece and sequentially making another clear with the next piece, and so on.
In some cases, drilling through garbage or skimming is likely to produce combos.
Not all games reward combos; those that do include the following:
Line clear gravity Edit
After a line clear, the blocks above the line move down.
How they move down depends on the game.
Some systems allow for "recursive gravity" that clears more than four lines with one piece.
Most tetromino based games use naive gravity. Here, the blocks above a cleared line move down by exactly the number of cleared lines below them. This can often leave floating blocks, unconnected to anything, after a line is cleared.
