A distinction made by Louis Hjelmslev, corresponds, at least roughly, to Saussure‘s distinction between signifier and signified. At the heart of each sign is (according to Hjelmslev) a correlation between a plan of expression and a plan of content. These are two distinct but inseparable aspects of the sign.
The plan of the signifiers is the plan of the expression, and that of the signifieds is the plan of the content.