Usually, you'll have a
<playfield> one or more lines of the same length </playfield>
<playfield> ..JJJ..... ....J..... .......... .......... .......... J.---...I. JJJZ-SLLI. OOZZTSSLI. OOZTTTSLI. GGG.GGGGGG GGG.GGGGGG </playfield>
It is possible to show one preview by putting the next piece all alone in a 2-row-high playfield above the main playfield.
Unlike the now-deprecated pfstart and friends, you can use any arbitrary width you want. Just be sure every row is the same length to ensure it works properly.
Note: Using space as an empty cell is not suggested, as it hinders readability for other editors; support for using space may or may not be unsupported in the future, as well. It is also suggested you only use purely uppercase letters, as there may be extensions in the future that interpret lowercase as something different from uppercase.
|Z||Red Block (Z Tetromino)|
|L||Orange Block (L Tetromino)|
|O||Yellow Block (O Tetromino)|
|S||Green Block (S Tetromino)|
|I||Cyan Block (I Tetromino)|
|J||Blue Block (J Tetromino)|
|T||Purple Block (T Tetromino)|
|G||Gray Block (Garbage or playfield wall)|
|-||Ghost piece or Line clear|
|C||Rotation center of a piece, a bomb in Bombliss, or any other specially marked block|
|P||Purple block for the T Tetromino, designed for use in documenting T-Spin setups|
|B||Mark a cell that has to be occupied for something (like a wallkick) to happen|
|X||Mark a cell that cannot be occupied for something (like a wallkick) to happen|
|1–9||Mark the cells that reject a given kick position|