Up to 1970, airliners were all "narrow-bodies", with usually 3 seats on each side of a single aisle. Then, in one year, three "wide-bodies", with two aisles, were put in service - the Boeing 747, the Douglas DC-10, and the Lockheed L-1011 Tristar. The latter was so technologically advanced for its time that some features it brought are not even common today.
Developed in the mid-1960s for transcontinental and transatlantic flights, it had around 250 seats, although some airlines managed to cram 380 people in them. It had extreme redundancy in every system - it had no less than four hydraulic systems, each with two powering systems, and could fly with only one functional. Every outside light was doubled, so it could still fly with one burned out.
The wing engines were mounted 10 inches further out than the DC-10 and had double enclosures, making it quieter than anything else then, and Eastern Airlines named it "Whisperliner". The tail engine was mounted aft of the cabin, and with an S-duct intake, in order to have a larger rudder than the DC-10 - tail height was limited by maintenance hangars.
The cockpit windows were curved (a rare feat even by today's standards) and are rumored to have costed $70,000 each, 40 years ago. They were so resistant there was no altitude, speed or cabin pressure limitation with cracked windows. Talking about windows, every single one in the cockpit and the cabin was 100% glare-resistant.
The cabin had full-size closets for coats that hid at the touch of a button, and galleys were below the deck and had elevators for the meals. The middle rows didn't have overhead bins, thus the ceiling was very high. The cabin air was completely changed every three minutes. The doors were electrically raised in the ceiling and had advanced system logic to deploy the slides automatically in an emergency. In the beginning, some airlines such as TWA had bars and lounges in the aircraft, and first class dinner was served on round tables that were propped up for the occasion.
But the most impressive feature was the very advanced, 100% fly-by-wire flight controls, that you find in the newest airliners. The autopilot used a new at the time inertial navigation system, and was the first aircraft to ever be certified to land automatically in zero-visibility blind conditions. It was so precise that Lockheed actually had to introduce a factor of error in its landing logic - the planes were all touching down within the same 6-by-6 inches area of the runways and were actually damaging them. This is years before GPS, and it was relying on radio beacons, gyroscopes and accelerometers.
On approach, pilots used a system called Direct Lift Control that maintained a steady pitch angle and used the inboard spoilers automatically to modify lift and maintain vertical speed control while keeping passengers comfortable. It was also very stable in rough turbulence due to its Active Control System. Accelerometers were installed in the fuselage and wingtips to detect vertical accelerations induced by turbulence or maneuvers. The system then used the information to symmetrically deflect the outboard ailerons in order to redistribute lift forces across the wingspan and to reduce wing bending. The result was decreased fuel consumption, and smoother rides in rough air.
Even with all this new technology, it maintained one of the best in-service rates of the time, at 98.1% reliability. Sadly, only 250 were made. Facing newer, more economical twin-engined widebodies such as the 767, Lockheed ceased production of the Tristar in 1984. Even today, pilots that got to fly it miss it dearly.