An example of bad user interface design. On my bus to Boston, the seats had headphone jacks with two volume controls. They were vertical thumbwheels with no marks or apparent indication of their range or current setting. Down was LO, up was HI, and the two wheels were marked LEFT and RIGHT. Entirely separate controls. So, turning it on you'd not only have no idea what volume would come out, you'd also have no idea what balance it would have (unless you first turned both wheels toward "LO" until they would go no further), and there usually be would be no way to equalize the left and right balance (without doing the same trick and then turning them both up, very carefully, simultaneously). Why would they design it like that? It was exposing a simple internal implementation of the more human-centered concept of master volume and balance. Maybe someone designed a single volume control then thought balance would be a great feature to add, but skimped on its implementation, exposed the internals, and ended up making it much harder to use for the most common case. There's a lesson in that for interface designers everywhere. And why have a balance control at all? You had to use the jacks with headphones, and headphones are not known for their widely varying distances from each ear.