Nepal is the only country whose national flag is not a quadrilateral.
Nepal’s flag consists of two joined triangular pennants, giving it a distinctive double-triangle shape. The upper triangle contains a stylized moon and the lower triangle contains a sun. A deep crimson field is edged in blue, colours associated with Nepal’s national identity and cultural traditions.
The current constitutional design was adopted in 1962, although the two-pennant form has older roots in South Asian banner traditions. The flag’s shapes are defined precisely in Nepal’s constitution, including geometric construction instructions rather than a simple rectangular ratio.
The distinction concerns the national flag of a sovereign country, not every flag used by a government, region or organization. Switzerland and Vatican City have square national flags in some contexts, but a square is still a quadrilateral. Nepal’s flag is therefore unique among national flags because its boundary is made from triangles rather than four sides.