Inverted flags[edit]
For hundreds of years inverted national flags were commonly used as distress signals.[4] However, for some countries’ flags it is difficult (e.g., Spain, South Korea, the UK) or impossible (e.g., Japan, Thailand, Israel and Denmark) to determine whether they are inverted. Other countries have flags that are inverses of each other; for example, the Polish flag is white on the top half and red on the bottom, while Indonesia's and Monaco's flags are the opposite—i.e., top half red, bottom half white. A ship flying no flags may also be understood to be in distress.[5]
If any flag is available, distress may be indicated by tying a knot in it and then flying it upside-down, making it into a wheft.[6]