Nepal is the only country whose national flag is not rectangular.
Nepal's flag consists of two joined triangular pennants, traditionally called a double pennon. The upper section contains a stylized moon and the lower section contains a stylized sun, both white and outlined in blue. Crimson red and deep blue are the flag's principal colors.
The current constitutional design was adopted in 1962, although its two-triangle form developed from older pennants used by Nepalese rulers and military units. The flag's unusual shape is not accidental: it preserves a traditional Himalayan banner form rather than adapting the common rectangular model.
Nepal's constitution also specifies geometric rules for constructing the flag, including mathematical instructions for the moon, sun, borders and triangles. Switzerland and Vatican City have square flags in some official contexts, but a square is still a rectangle in geometry. Nepal's flag is therefore the sole national flag with a fundamentally different outline.