Fly over most of the world's political borders and there's nothing to see — no line painted on the ground, no natural feature that marks where one country's authority ends and another's begins. A political border is a human agreement about where sovereignty stops, and that agreement has to be actively maintained, because the land itself gives no indication that anything changes at all.
Borders that follow rivers and mountains, and borders that follow nothing at all
Geographers usually split borders into two rough categories. A "natural" or physical border follows some feature of the landscape — a river, a mountain ridge, a coastline — which sounds simple but frequently isn't: rivers shift course over decades, leaving the legal border stranded on dry land or in the middle of a new channel, and a treaty specifying "the thalweg" (the deepest channel) of a river can become a genuine dispute once the river itself has moved. An "artificial" or geometric border, by contrast, follows a straight line of latitude or longitude or a ruler's mark on a map with no relationship to the terrain at all — much of the western United States–Canada border, and many African and Middle Eastern borders drawn during colonial negotiations, were set this way, often by people who had never visited the land they were dividing.
A line is only a border once enough people stop contesting it
What actually makes a line a functioning border isn't the line itself but recognition — by the two states sharing it, and usually by the wider international community. A border can be drawn on official maps and still be actively disputed on the ground, with both sides maintaining rival claims, competing administration, or ongoing military presence, which is why political geographers distinguish a formally agreed, demarcated boundary from a merely claimed or contested one. Many of today's most stable-looking borders were, within living memory, exactly this kind of unresolved line — what changes them from contested to settled is usually a treaty, a war's outcome, or decades of both sides simply not pressing the issue rather than any change in the land itself.
What we're still unsure about
Dozens of international borders remain actively disputed today, and geographers and international law scholars don't have a clean, universally agreed method for resolving competing claims when historical treaties are ambiguous, populations on the ground don't match the political division, or the original colonial-era documentation is incomplete or contradictory. Where a border "should" go when ethnic, linguistic, and historical claims all point in different directions is a political and legal question that geography alone can describe but can't settle, which is why a meaningful share of the world's land borders remain formally unresolved.
This sits inside Political Geography & Territorial Organisation, one of seven topics in Human Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.