Modern airliners are usually low-wing designs with engines mounted in underwing pods (usually two of them). For airliners, multi-engine design is mandated by some national regulations so that aircraft can continue to climb even in the worst case of power loss in one engine right after take-off. Another regulatory demand is that aircraft are able to fly a minimum specified amount of time after one engine fails in flight.
The idea of mounting the engines underneath and to the fore of the wing was first implemented by Boeing with the B-47 bomber of the 1950s, with the realization that this would provide for lesser efforts on the wings and therefore allow for a lighter wing structure. After this feature proved successful, Boeing introduced it to its 707 airliner design and it has been increasingly adopted since.
Mounting the engines in underwing pods also makes physical access for maintenance quicker and easier compared to tail-mounted engines.
Additionally, low wing design helps keep the engine nacelles and refueling valves closer to the ground to simplify access and the wing's surface acts as a barrier to prevent the engines' noise from reaching the fuselage in-flight.
Both Airbus and Boeing use this common layout for all of their passenger aircraft and emerging manufacturers (e.g. Embraer and Sukhoi Superjet) follow the same scheme.
In a few special cases, where engine proximity to ground is detrimental (e.g. rural airfields with risk of foreign object damage or dirt), airliners will feature tail-mounted engines (e.g. MD-80 or Tu-334) or high-wing designs with underhung nacelles (e.g. BAE-146). These planes become rarer every year as almost all newly-built airliners have underwing nacelles. Tail-engined designs are mostly used by business jet manufacturers.
Future airliners may feature innovative delta-wing or lifting body outlines.

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