The only current national flag to not be rectangular or square is that of Nepal.
The two flags that are square in shape are the flag of Switzerland and the flag of Vatican City. The Swiss flag consists of a red field with a white cross in the center, while the flag of Vatican City features two vertical bands of gold and white with the crossed keys of Saint Peter and the Papal Tiara at the center. Both flags are unique in their design and are easily recognizable due to their square shape.
It is a rectangle.
That of Nepal.
Nepal, the flag is two combined triangular pennons.
the flag is nepal. its like two triangles put together on top of each other
Ohio uses a pennant shape.
Sure, honey, let's break it down. Since the flags are similar, their corresponding sides are in proportion. The ratio of their areas is the square of the ratio of their heights. So, if the larger flag's height is 180cm, the smaller flag's height would be 120cm (180cm * sqrt(0.8/5) = 120cm). Voilà!
None of the flags are square; they're rectangular. Except for Nepal.
No, Nepal's flag is the only none rectangular national flag, with the exception of the square flags of Switzerland and the Vatican City. The flag and more details are shown on the related link.
The only current national flag to not be rectangular or square is that of Nepal.
Square
Yes, although they're not required, there aren't any countries that don't have an official flag. The biggest stand-out is Nepal which has a flag, but it's not rectangular like all the other countries' flags are.
There is no specific reason, actually. Also, I know at least one flag does not; look up Ohio's flag. (alternate answer) Rectangles are a convenient shape to work with. You might also ask why pieces of paper are rectangular, books are rectangular, newspapers and magazines are rectangular, candy bars are rectangular, and so forth. It would be trickier to make pentagonal flags. Why give yourself extra work? Switzerland's flag is a square rather than a country; I think they had to get special permission for this. Rectangular flags appear to be golden rectangles which are said to look nicer, so perhaps this has something to do with it.
The flag of Nepal is the only national flag that is not rectangular in shape. It consists of two stacked triangular shapes.
Nepal
Nepal is the only Asian country without a rectangular flag. Instead, it's flag is in the shape of two overlapping triangles.