Small-screen layout
The zoom layout is better suited to small-screen devices, such as PDAs and smartphones, and may also be useful if you browse with very large text, since a single column gives more room for scaling without creating horizontal scrollbars.
If your window is 480px or less then the small-screen layout should kick-in automatically — for example, if you're using an iPhone.
You can change the layout and colour scheme at any time, using the Stylesheet switcher (which can also be found on the home and accessibility pages).