The philosophical tradition has been fundamental to what's called truth-conditional semantics. Truth-conditional semantics views the meaning of a sentence as the set of worlds in which the proposition it expresses is true. This is why different sentences that express the same proposition will have the same truth-conditions and therefore the same semantic meaning. One way to think about the meanings of words is by considering how they relate to other words. There are many ways in which words can be related. This chapter considers a few relations, including synonymy, antonymy, hyponymy, and homonymy. Just as one way to think about the meanings of words is through their relations to other words, one way to think about the meanings of sentences is through their relations to other sentences. The chapter points out the commonalities between lexical semantics and sentential semantics largely because it helps one to see how the meanings of words contribute to the meaning of the sentences containing them.