Korean text can be written using a phonetic writing system called Hangul. Hangul has more than 11,000 characters, which consist of consonants and vowels known as jamos. About 3000 characters from the entire Hangul vocabulary of characters are usually used in Korean computer systems. Korean also uses ideographs based on the set invented in China, called Hanja. Korean text requires over 6000 Hanja characters. Hanja is used mostly to avoid confusion when Hangul would be ambiguous. Hangul characters are formed by combining consonants and vowels. After these characters are combined, they can compose one syllable, which is a Hangul character. Hangul characters are often arranged in a square, so that the group takes up the same space as a Hanja character. Arabic numerals, Roman letters, and special symbol characters are also present in Korean text.