Rear-end

This is the rear end of a computer - I was trying to see if the green jack was plugged into the correct socket. It was. However the other end of the cable wasn't plugged into what I thought which is why it didn't work.

This picture has the green cable in the wrong position, which was one of several I tried, until I took this picture and could see what was wrong - or not. Once I had the jack in the correct socket, I was even more puzzled until I figured out what was going wrong at the other end...