If you're getting Di2 for your road bike, you're going to need a Shimano E-Tube Di2 3_port A-Junction Box. What the heck is that?! It's the part that you plug your shifters in to so they can talk to your rear derailleur. Technology! It is designed for a clean installation under the stem and features a plug on the side for charging an internal battery if you have one. How neat is that?!
designed for clean installation under the stem
3 ports allow for the connection of 2 shifters and the rest of the system; best used for road bikes
has a plug on the side for charging an internal battery if so equipped
not compatible with 7970 Di2 systems