My guess would be the desktop resolution? I designed my layout based on percentages, so it will dynamically fit any resolution. Some people design for a set of specific resolutions. Some have only 1 hard set resolution. Everyone seems to be taking a different approach.
I should design some resolution helpers to determine if aspect is widescreen or standard, and people could use percentages and have certain variables change based on format. It could be as simple as using a tenery expression.