Today's computer displays come in two aspect ratios: 4:3 (standard) and 16:9 (widescreen). This means that the vast, vast majority of computer monitors are in landscape orientation. That is, where the width is greater than the height. The reasoning for this is a bit uncertain, but it likely has something to do with the fact that this is how our TVs are oriented. But this is actually rather counterintuitve—after all, we use our computers to view much different types of content than we do with our TVs.
It's a little known fact many monitor drivers support portrait orientations, with aspect ratios of 3:4 or 9:16. While your co-workers may look at you strangely when they see your monitor turned “sideways,” there are a number of benefits to using your monitor in portrait mode.
See More, Scroll Less
Think about the content that you view on a regular daily basis. Web pages, Word documents, PDFs, spreadsheets, etc. All of these scroll vertically as we read on. With a portrait-oriented computer monitor, more of the content will appear on your screen at a time, meaning you'll have to scroll less. This can actually improve your reading speed, since it takes a few split seconds for you to get your bearings each time you scroll a page a notch.
Print Previews in Actual Size
Considering the A4 standard page size (8 in. x 11 in.), most of the documents that we author using desktop publishing software is portrait oriented. With a landscape oriented monitor, zooming out to fit the document to the page leaves you with excess space on the left and right in order to squeeze the vertical area into the viewing area. With a portrait-oriented screen, if your monitor is at least the size of a sheet of paper (and all desktop monitors are) you can view any full page document in its actual size before printing.
Toolbars in their Right Place
Most applications place their toolbars and other menus along the top of the screen. On a landscape oriented monitor, this eats up even more of your vertical screen real estate. But when you set your monitor's resolution to portrait, you'll have space to spare. Plus, if you choose to move your Windows taskbar to the top of the screen, it'll be closer to eye level, which can help you monitor notifications in the taskbar and system tray.
Mobile App Development
Mobile apps on smartphone platforms are designed for both portrait and landscape orientations. It's possible that this trend will carry over to the desktop computing space, especially considering the wider selection of pivoting computer monitors entering the market.
At the end of the day, it often makes more sense to view your computer documents and applications in portrait mode. With the advent of Internet connectable TVs and streaming media to set-top boxes, our multimedia experiences are migrating away from the PC and back to the home theater. As such, it's less logical for computers to mimic widescreen TVs.