Nintendo's Game Boy was essentially an expansion on the idea of their Game & Watch series. Game & Watch were handheld units featuring an LCD screen, a four-button directional pad, and one or two action buttons, each programmed with a single game. Game Boy did for handhelds what Atari 2600 had done for home consoles: introduced exchangeable cartridges, making it possible to play a variety of games on a single portable system. Game Boy was originally released in 1989, and featured the 8-way directional pad, A, B, Start and Select buttons, familiar from the NES control pad. Part of Game Boy's overwhelming success can be attributed to Tetris, which came with the unit in the U.S. Simple graphics and addictive game-play made Tetris the perfect time-passing game for adults and children alike, and soon everyone was buying a Game Boy as a travelling companion.