When we look up at a tall building its sides slope inwards to form an inverted V shape. We are so used to seeing in this way that we hardly notice the effect while we are enjoying the view. When we take in the same scene in 2 dimensions, as we do when looking at a photograph, the experience is quite different and we do notice the convergence of the vertical aspects of the building.
If the effect is exaggerated, or at least very obvious, it lends a sense of height and drama to the picture, and we can appreciate a a split second just how tall that building is. However, if the effect is only moderate the building might look as though it is tilting backwards and in danger of falling over.
As with many photographic visual effects moderate applications simply look like mistakes, so we should avoid them at the shooting stage, or learn to correct them afterwards.
The way to avoid the problem is to hold the camera completely level when taking the picture, as it is the looking up angle of the camera that creates the effect. Unfortunately, keeping the camera level usually means that the top of the subject will be cut off. You can move backwards to alter your perspective, but in the majority of cases this is not an option as space is usually limited in architectural locations. In any case you’d have to move a long way for even a moderately tall structure