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
Single
A single is the act of two lines coming together.
Double
A double is when two lines are cleared at once:
Triple
A triple is three lines cleared simultaneously. Most games allow only I, L, and J tetriminos to complete a triple, but newer games with SRS allow S, Z, and T tetriminos to twist into seemingly impossible positions.
Tetris
A tetris is four lines cleared simultaneously. In most games, this can only be done with an "I" tetrimino.
Jeem/Hah/Khah
Jeem refers to clearing five lines, Hah refers to clearing six lines, and Khah refers to clearing seven lines. This is possible in games of heptiminos such as Seentis.
The scoring system for games supporting JHK is as following:
Single: 1 point
Double: 3 points
Triple: 5 points
Tetris: 8 points
Jeem: 9 points
Hah: 11 points
Khah: 13 points
Back to back Tetris: 12 points
Back to back Jeem: 12 points
Back to back Hah: 14 points
Back to back Khah: 18 points
Tetris Plus
A Tetris Plus occurs when five or more lines are cleared simultaneously. This can be done with an "I" pentamino in Party Mode or by stacking multiple tetrominos on top of each other. The latter can be performed by allowing a tetromino to fall naturally onto a stack; which causes the next piece to spawn while the previous piece is still falling; pieces can then be dropped onto the still-falling tetrimino to "stack" them.
This can also be done in games with cascade gravity.
Hurdle or Split
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-Back
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 them a Back-to-Back tetris.
Combo
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
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.
Naive
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.
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Drop J
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Line clear
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Naive gravity
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Sticky
The playfield is divided into connected segments using flood fill. Any blocks that are adjacent horizontally or vertically are marked as one segment, that is, they are treated as having "stuck" together. Each segment falls independently until it meets the floor or another block. Additional line clears may result.
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Drop J
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Line clear
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Mark segments
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Segment  lands
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Segment  lands
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Line clear #2
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Mark segments
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Segment  lands
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Games that use sticky gravity:
Cascade
When each piece locks, its connections to other blocks in the piece are stored. After lines are cleared, each piece is marked as a separate segment, and then each segment falls independently until it meets the floor or another block, as in Sticky.
Games that use cascade gravity:
Non-tetrimino games:
Delay
Some games impose a line clear delay after each piece that completes one or more lines and/or a line gravity delay every time blocks move down by one row.
For example, Tetris The Grand Master and Tetris DS wait 400–700 ms.
In games with a large line clear delay and scoring based on play time, it is to the player's advantage to make multiple lines at once (triple or tetris) so that less time is spent in line clear delay.
See Also
