Chinese characters are logographs used to write the Chinese languages and others from regions historically influenced by Chinese culture. Of the four independently invented writing systems accepted by scholars, they represent the only one that has remained in continuous use. Over a documented history spanning more than three millennia, the function, style, and means of writing characters have evolved greatly. Unlike letters in alphabets that reflect the sounds of speech, Chinese characters generally represent morphemes, the units of meaning in a language. Writing a language's vocabulary requires thousands of different characters. Characters are created according to several principles; aspects of shape and pronunciation may be used to indicate the character's meaning.