Here, you can clearly see the top of the applications circuit board (1) and the top of the wireless interface board (3). In the middle, you see the inside of the iPhone's back surface. The 3.5mm stereo headphone minijack and related circuitry (2) is crammed into the upper left corner. Although the jack itself is standard, when the chassis is assembled, the jack is actually inset a bit, making it difficult for most ordinary headphones to fit into the tight space. If you want to use your current headphones on the iPhone, you'll need to buy a $10 adapter.