A folder in the site architecture that is dedicated to holding mobile versions of each page is sometimes called a mobile sub-directory. Mobile sub-directories can be anything, but are often just ‘m’ or ‘mobile’. On an example domain, example.com, a mobile sub-directory might be expressed as https://example.com/m/ or https://example.com/mobile/. This is different from a mobile sub-directory, which is hosted in an entirely different part of the server. Sub-domains are expressed before the name of the domain, as in ‘https://m.example.com/ or https://mobile.example.com. In either case, if there are separate pages used for mobile content, it is important for SEO that you include bi-directional tagging to help Google understand which mobile pages match to which desktop pages.