Answer:
The flag of Switzerland is square.
Step-by-step explanation:
Only Switzerland has a flag that is square.
The measurements can be anything convenient. The aspect ratio (height to width) is 1 : 1.
When the flag is displayed next to a rectangular flag, it should have the same area as the rectangular flag. (This will mean the Swiss flag side length is the geometric mean of the rectangular flag dimensions.)
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Sometimes, the measurements of the flag are chosen to go with the height of the flagpole. The US flag is customarily displayed on a pole 3-4 times as long as the flag is long. That is, the diagonal of the flag is about 0.28 to 0.38 times the pole height.* I could not find comparable information about the Swiss flag.
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* The diagonal is of interest when the flag is hanging down along the pole, not flying in the wind.
