Most of the early games were based on traditional real-world concepts such as sports, with an emphasis on realism and depth of play within the technology of the day. The Intellivision was not marketed as a toy; as such, games such as "Sea Battle" and "B-17 Bomber" are not made in a pick-up-and-play format which arcade style games are. Reading the instructions is often a prerequisite to play. Every cartridge produced by Mattel Electronics included two plastic controller "overlays" to help navigate the 12 keypad buttons, although not every game made use of the keypad. Mattel organized its games into networks: "Major League Sports, Action, Strategy, Gaming, Children's Learning" and later "Space Action" and "Arcade". The network concept was dropped in 1983, as were the convenient gate-fold style boxes for storing the cartridge, instructions, and overlays.