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Ovals and circles are fairly common in Korean script but virtually unused in Chinese or Japanese characters. If you see a lot of open ovals and circles, as well as open squares, you can be fairly certain that you are looking at Korean script.  While Japanese has curvy shapes, it doesn't have complete circles within the characters as Korean does. Korean has an alphabet, just like the English language. However, the written language combines 2 or 3 letters into a single character representing the syllable of a word. So you'll also see strong vertical or horizontal lines separating the letters. Chinese characters (called hanzi in Chinese and kanji in Japanese) may appear in Chinese, Korean, or Japanese. However, if the writing is in nothing but Chinese characters, then what you're looking at is Chinese. It is the only language of the 3 to rely solely on hanzi.  Chinese characters are extremely detailed. Strokes don't cross outside the square perimeter of any character, giving a line of script a consistent, uniform appearance. In contrast, Japanese has a more open and airy style of script. While Korean characters are orderly and regimented, they aren't quite as dense or complex as Chinese characters, with a lot more open space. Loose, curvy lines that aren't constrained to a small box are Japanese hiragana. The characters are extremely simple compared to Korean or Chinese characters, some only requiring a single stroke of the pen to reproduce. For a quick shortcut, look for the character "の." Neither Korean nor Chinese have a character that looks anything like "の." It's typically pretty easy to remember and spot. If you see this character in a passage of writing, it's most likely Japanese writing, even if you notice a few Chinese characters.
Look for circles and ovals to identify Korean writing. Recognize complex square characters as Chinese characters. Identify Japanese script as light and loose.