Why are commercial aircraft boarded from the left?

Before jetways, generally the aircraft taxied in with the left side toward the terminal (American, DC-3 doors on the right) did it backwards. This was so the Captain could see the obstacle better, later jetways showed up there.

Actually, you have your hands mixed up there! :-) We board on the left. But we know what you mean!

Do you know, in about 50 years of flying, I have never, ever, thought about this! It has always been the way it is!

I guess that it places the boarding situation on the Captain's side of the aircraft and of course, since the advent of airbridges it makes them all the same.

Many aircraft only have ground steering tillers on the left hand side, so that might have something to do with it. Much easier to bring the aircraft alongside the airbridge if you can see where you are steering.

I guess it is a case of, there is no reason for it, it is just our policy!

In modern large aircraft, the captain does all the taxiing because only he has access to the steering tiller. Because the captain is seated on the left, he can only see well out the left side. Therefore all jet-ways are oriented on the left side and the left door is used. When an air stair is employed, it keeps exiting passengers away from the baggage carts and service trucks that are almost always on the starboard side of the aircraft.

The earliest airliners were flying boats, and for practical convenience they followed the shipping convention of boarding from the port (left) side.

Having ships/airliners boarding from one common side makes port/airport manoeuvres simpler and safer; and keeps the starboard side of an airliner free for the potentially hazardous operations of loading luggage and refuelling which it's best to keep passengers safely away from.