What is Ferdinand de Saussure’s linearity principle?

See e.g. http://personal.bgsu.edu/~dcalle… : Principle II: The Linear Nature of the Signifier

The linearity principle is Saussure’s statement that, because linguistic signifiers are sounds (spoken words), they are intrinsically sequential (“linear”). They cannot be perceived simultaneously, the way visual signs are: they must be perceived one after the other, as a sequence in time. That principle is also carried over to writen words, as a visual representation of spoken words.