Tetris Wiki

Nearly all falling block games are capable of showing a preview of the next piece to enter the playfield. The next pieces are displayed in a window of the tetrion, generally positioned to the right of or above the playfield.

OO
OO

Game start

J
JJJ
OO
OO

O enters; J is next

J
JJJ
OO
OO

Player drops O

T
TTT
J
JJJ
OO
OO

J enters; next is T

T
TTT
JJ
JOO
JOO

Player drops J

ZZ
ZZ
T
TTT
JJ
JOO
JOO

T enters; next is Z

Some of the older games gave more points if the player chose to turn preview off.

Many Tetris games released since 2001 show 1 to 6 next pieces.