Before responsive web design existed the easiest way to have a mobile friendly site was to have two separate websites, a desktop site and a mobile site. Primarily the mobile site would live on an m.website.com domain or a /m or /mobile domain. One of the main issues with this having to redirect all of your desktop URLs to your mobile URL. No one likes trying to go to a specific page on your desktop site, only to be redirected to the mobile homepage, having to find your way back to the content you were looking for. Sadly, many search engines still don’t rank mobile sites high enough for mobile users, so people are often guided to full sites instead of the mobile ones, which offer a superior user experience.