Which national flag is the only one in the world that is not a quadrilateral?

Nepal’s flag is the only national flag in the world that is not a quadrilateral.

It consists of two joined triangular pennons, outlined in blue and filled with crimson red. The upper pennon contains a stylized moon and the lower pennon a stylized sun. The red represents Nepal’s national flower, the rhododendron, while blue symbolizes peace and harmony.

The modern constitutional design was adopted in 1962, although the two-pennon form developed from older South Asian banner traditions. Its precise construction is defined geometrically in Nepal’s constitution, making the flag unusually specific in its official proportions.

Many flags have distinctive shapes without breaking the quadrilateral rule. Switzerland’s flag is square, and the Vatican City flag is also square, but both are still four-sided. Bhutan’s flag is divided diagonally and Sri Lanka’s contains elaborate imagery, yet each remains a quadrilateral.