Sometimes we don't have a horizon line to keep everything grounded, so we have to pick a straight line to balance a photo. We call this an Anchor Line, because it's what keeps the photo still, upright and from going wonky. There are two main times you will use Anchor lines in a photo...
The first situation is when you are shooting a scene at an angle other than front on, i.e. from one corner of a room to another, like in the photo above. When we are shooting a scene like this, we think of the Anchor Line as a tent pole - the one straight vertical line that all the other lines can hang off. Usually we pick the biggest straight vertical line in these situations. It could be the line of the corner of a room like it is above, or it could be something like a vertical beam or fence post.
The second situation we use Anchor lines is when we ARE shooting straight on, but the lines in the scene are all wonky and impossible to have lined up parallel to one another. This often happens when shooting old buildings and naturey stuff. In this situation we picture a set of old fashioned scales, and tip the scene back and forth a little bit on our camera until we feel like things seem as balanced as they can get. Often it's a matter of picking one line in the scene, and having the other lines fall off in other directions. In this old Spanish building there were no totally straight lines and the street was on a slope, but Kate felt that having the vertical lines in the windows straight gave the most sense of balance. It doesn't have to be a vertical that you go with, sometimes it may be a horizontal line that gives the scene that sense of balance you are looking for.
Get out your tent poles or your scales! (Not literally!) Time to use an anchor line in a photo!