“D” system is the most common system available in the US, also known as American Sankey. It is used by all the major breweries – Anheuser-Busch, Miller, Molson-Coors, and Labatt as well as most craft breweries.
The keg coupler connects to the keg valve, “tapping” the keg. Both the gas line from the CO2 tank / regulator and the beer hose to the beer faucet attach to the coupler.