Which country's flag is the only non-rectangular national flag?

Nepal, whose flag is two stacked pennants - and it is the only national flag that is not a quadrilateral. Its construction is specified in the constitution with a page of geometric instructions involving compass arcs and intersections, so the shape is defined by drawing procedure rather than by proportions.

The two triangles are usually explained as the Himalayas or as the two dynasties that combined, and the emblems inside them are the moon and the sun - traditionally signifying a wish that the country endure as long as they do. The current form dates to 1962, but the double-pennant shape goes back centuries.

Nepal is also the only country never colonised in South Asia, which is not unconnected to the terrain, and it contains eight of the world's ten highest mountains. Its flag is a genuine nuisance internationally: rows of flags at the United Nations and at sporting events are designed around rectangles, so Nepal's is either hung with empty space beside it or, occasionally and incorrectly, printed on a rectangular field.