Only the proposition has sense; only in the context of a proposition has a name meaning.

3.31 (8)    Every part of a proposition which characterizes its sense I call an expression (a symbol).

(The proposition itself is an expression.)

Expressions are everything - essential for the sense of the proposition - that propositions can have in common with one another.

An expression characterizes a form and a content.

3.32 (8)    The sign is the part of the symbol perceptible by the senses.

3.33 (4)    In logical syntax the meaning of a sign ought never to play a role; it must admit of being established without mention being thereby made of the meaning of a sign; it ought to presuppose only the description of the expressions.

3.34 (4)    A proposition possesses essential and accidental features.

Accidental are the features which are due to a particular way of producing the propositional sign. Essential are those which alone enable the proposition to express its sense.