Thought expresses itself perceptibly in a sentence.

We use the perceptible sign (sounded or written etc.) of a sentence to project a possible situation. That projection is accomplished by thinking the sense of the sentence. Wittgenstein calls a sign which expresses a thought a propositional sign. And so, a sentence is a propositional sign that relates to the world in a projection.

A sentence includes all that is needed to project, but not what is projected. That is, the possibility of what is projected, but not that itself. So a sentence does not already contain its sense, but does contain the means to express that sense. ("The content of the sentence" means the content of a sentence that one can make sense of.) A sentence includes the form of its sense, but not the sense itself.

The propositional sign consists of its elements, words, which relate to each other in it in a definite way. The sentence is articulated; it is not a medley of words. (Just as a musical theme is no medley of tones.)

The propositional sign is a fact. Only facts can make sense, a class of names cannot. That the propositional sign is a fact, is obscured by its usual form as written or printed. (Thus it was possible that Frege called the proposition a compound name.)

The nature of the propositional sign becomes very clear if, instead of lexical symbols, we think of it as composed of material objects (such as tables, chairs, books). The spatial relation of these things then expresses the sense of the proposition.

Not: "The complex sign 'aRb' says that a is in relation R to b." but: That 'a' is in relation R to 'b' says that aRb. One can describe situations, not name them. (Names are like points. Sentences are like arrows; they have a sense.)