The theoretical linguistics constructed by the Swiss Ferdinand de Saussure is credited with paving the way for structuralism, semiology and semiotics. In his posthumously-published Course in General Linguistics (1916), Saussure theorized that language should be treated as a system of signs. Each of these signs is in a structured relation with the others and can be understood only through this relation. A sign is formed by a "signifier" or "sound-image" (the material letters and sounds that form the word on the page or in an utterance) and a "signified", the mental concept that the "sound-image" evokes. For example, the letters and sounds in "a-p-p-l-e" evoke the signified "green or red fruit". Thus, to Saussure, the relation between signifier and signified is entirely arbitrary: language is a social convention, where a given sign gains meaning through its relationships with other signs.