As it has been for a very long time, road and mountain bike drivetrains have used two chain-rings up front. This was a design necessity to accommodate the gear-ratios previously unusable with a single chain-ring. SRAM flipped that by introducing a 1X drivetrain for MTB last year. The system uses a single front chain-ring and provides wider cassette gear-ratios that reduce weight and complexity while increasing user experience.
Earlier this year, SRAM released 1X road drivetrains with similar success. For many riders, the ratios available provide enough of a range to tackle even the surliest of grades. For Shimano riders anxious to try a a 1X drivetrain, this might be the ticket.