In the past many borders were not clearly defined lines, but were neutral zones called marchlands. This has been reflected in recent times with the neutral zones that were set up along part of Saudi Arabia's borders with Kuwait and Iraq (however, these zones no longer exist). In modern times the concept of a marchland has been replaced by that of the clearly defined and demarcated border.

For the purposes of border control, airports and seaports also class as borders. Most countries have some form of border control to restrict or limit the movement of people, animals and goods into or out of the country. In order to cross borders people need passports and visas or other appropriate forms of identification. To stay or work within a country's borders aliens (foreign persons) may need special immigration documents or permits that authorise them to do so.