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Ferdinand de Saussure theorizes that a "sign"[clarification needed] has two parts: signifier and signified. The signifier is the form that the sign will take, whether it be a sound or image and the signified is the meaning that is conveyed.
Signifier: the word 'tree' = Signified: the mental image of a tree.
Charles Sanders Pierce analyzed sign systems and came up with the following: An Icon signifies by resemblance. An Indexical sign signifies by causal connection and finally, a Symbol signifies by learned convention.