Line clear

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.

Single
A single is the act of clearing one line at a time:

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 tetrominoes to complete a triple, but newer games with SRS allow S, Z, and T tetrominoes 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" tetromino.

Hurdle / 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 him a Back-to-Back tetris.

Line clear gravity
After a line clear, the blocks above the line move down. How they move down depends on the game.

Naive
Most Tetris 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. Most tetromino based games use naive gravity.

Sticky
The playfield is divided into connected segments using flood fill. Any blocks that are adjacent horizontally or vertically are marked as one segment. Each segment falls independently until it meets the floor or another block. Additional line clears may result.

Games that use sticky gravity:
 * Super Scope 6 Blastris A
 * Tetris Blast and Bombliss
 * The Next Tetris (modifies rule such that only blocks of a single color can form a segment)
 * Re-released as Tetris Worlds Sticky
 * Lockjaw: The Overdose

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:
 * Tetris 2 (Tetris Flash)
 * Quadra
 * Tetris Worlds Cascade and Fusion
 * Tetris DS Touch

Non-tetromino games:
 * Dr. Mario

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 about 667 ms + 16 ms per row. 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.