 An important consequence of the principle of compositionality is that all the parts that play a role in the syntactic composition of a sentence must also have a meaning. And furthermore, each syntactic rule must be accompanied by a semantic rule which says how the meaning of the compound is obtained. Thus, the meaning of an expression is determined by the way in which the expression is formed, and as such the derivational history plays a role in determining the meaning. For further discussion, see Section  2.5.