Some states separated by the borders with high fences, other make a simple colored line between them, and some are separated by natural landscapes. Examples of such limits in this post.
The boundary between Belgium and the Netherlands
The boundary between the United States (Arizona) and Mexico. Is a game of volleyball
The boundary of three countries: Argentina, Paraguay, Brazil
The boundary between Mexico and the United States. Photos of the first half of the twentieth century
The boundary between Iraq and Syria
The boundary between Italy and Switzerland, at an altitude of 3470 m
The boundary between Austria, Slovakia and Hungary
Beer cafe, through which the border between Belgium and the Netherlands
The boundary of Germany and the Czech Republic
U.S. and Canada
Ukrainian-Polish border
Bangladesh and India
Russia and Belarus
As a bonus: the border of the Black Sea and Sea of Azov