Rigid-Flex PCB: A Genius Mix of Rigid PCB and Flexible PCB

Once, a guy left a comment that said the above circuit boards were merely rigid FR4 PCBs connecting with soft adhesive.

Well, that's wrong. These circuit boards are rigid-flex PCBs, also known as semi-flex PCBs.

On the whole rigid-flex board, you can design any areas to be bendable and any other places to be rigid. Circuits are interconnected in the whole rigid-flex PCB, and you don't need any independent connectors or jacks to take up the compact space anymore!