squares once they have landed. Grouped blocks have a 2×2 shape and vary between two colors. Players can rotate the descending grouped blocks, move them left or right, or drop them straight down. When part of the grouped blocks hits an obstruction, the remaining blocks separate from the rest and continue to fall. A single-color square is created when grouped blocks form a 2×2 shape of matching color. Additional blocks of matching color can be used to create larger shapes. The game ends when the blocks pile up to the top of the playing field. A vertical line known as the "time line" sweeps through the playing field from left to right, erasing any completed single-color groups of blocks it touches and awarding points for each group. If a square is created in the middle of the moving time line, only half of the square is taken and no points are awarded. Players earn increasing score multipliers by repeatedly clearing squares on consecutive time line sweeps. Additional score bonuses are earned by clearing the playing field or reducing the remaining blocks to a single color. The maximum score is 999,999 points. Blocks with embedded gems are known as "special blocks"; if these are used to create squares, they allow the time line to eliminate adjacent blocks of the same color. Each stage has a skin that affects the background's appearance, the blocks' color scheme, the music track, sound effects, and the speed of the time line. Stages with fast tempos make it more difficult to create large combos and stages with slow tempos may cause the playing field to fill more quickly while players wait for the time line to sweep across the screen. There are five modes: Challenge, Single Skin, Time Attack, Puzzle, and Versus. Challenge mode is the main mode and cycles through skins in a fixed order of increasing difficulty. Single Skin mode allows players to select one skin to play for the entire session. In Time Attack mode, players have a limited time to clear as many blocks as possible. Puzzle mode challenges players to arrange blocks to create pictures. In Versus mode, players battle against A.I. opponents or against other players via their wireless connections. The Versus mode begins with the playing field divided in half; the goal is to clear successive squares, which shrinks the opposing player's space. Later ports and remasters added modes