This is a question from a mathematically-inclined homeowner who knows he /should/ be able to figure it out, but can't. I need to hang a heavy mirror in a house in Berkeley, CA, which is a seismically-active area. The mirror has a cable on the back of it attached to two sides of the mirror (just like a cable for hanging a picture). The nails need to be placed in studs in case of an earthquake. The problem is that the mirror needs to be centered on the wall--but the studs aren't centered on the wall. Therefore, I plan to use two off-center nails, placed at different heights, to hang the mirror so that it is level. The final configuration is shown in the attached image. My question is: given the two distances from each mirror edge to the nail locations (x and y in the diagram), how can I figure out the difference in the height of the two nails that is required to make the mirror level? Or, if you can't tell me this, can you just tell me how to think about the problem? I don't get how to figure out the amount and direction of tension placed on each cable terminus, given the fact that they are connected to each other by the mirror itself. If I knew this, then I suppose I'd just need to place the same amount of vertical tension on each cable, and I'd be done.