Borders are agreed upon boundaries that mark the perimeter of a country or territory. They often fall on difficult to cross or easy to defend terrain features such as rivers or mountain ranges. In more fantastic worlds giant canyons, magical rifts, stormlands, or other regions may also be places a border may be located. Borders between countries may have watchtowers, border towns, border checkpoints, great walls, or minefields. The surrounding lands may be borderlands or a frontier. There may be contested territories or neutral zones nearby. Borders are home to smugglers, soldiers, and traders.

Borders often fall on natural terrain features that are easy to defend, difficult to cross, and are convenient to identify by word of mouth or on maps. For worldbuilders this means placing borders on terrain like rivers, mountain ranges, and coasts. Other terrain that fits these qualifications, such as great chasms, canyons, or expansive deserts, are additional options.

Modern borders are less constrained by natural terrain features. Modern, more precise, methods of surveying the globe meant borders could be drawn along specific latitudes, longitudes, or between known sites. This means that straight borders with no regard for the local terrain could exist. This does not mean that straight borders should not exist in worlds of an older era, simply that they may be more common in more modern times.