all the above
By international agreement. They may include physical landmarks such as rivers or mountains.
Idaho's eastern border is defined by the rocky mountains.
Scotland and England share a common border.
GuatemalaBelize
Mexico/Canada
Tables
A border is a physical or political boundary that separates two countries. It can be defined by natural features like rivers or mountains, or it can be man-made, such as a fence or wall. Borders are important for defining the territorial limits of a country and regulating movement of people and goods between nations.
Only one nation shares a common border with the USA's southern border: Mexico.
The southern border of Ohio is 981 miles and is defined by the Ohio River. It is also the southern border of Indiana and Illinois.
Lake chad and Spain
Mongolia and Kyrgystan
North Korea, Russia