From its introduction, Coleco had touted a hardware add-on called the Expansion Module #1 which made the ColecoVision compatible with the industry-leading Atari 2600. Functionally, this gave the ColecoVision the largest software library of any console of its day. The expansion module prompted legal action from Atari, but Atari was unable to stop sales of the module because the 2600 could be reproduced with standard parts. Coleco was also able to design and market the Gemini game system which was an exact clone of the 2600, but with combined joystick/paddle controllers.
Source: Wikipedia, "Coleco Vision", available under the CC-BY-SA License.