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While explaining the problem of what philosophy is according to Wittgenstein's Tractatus, Frank P. Ramsey says:
a propositional sign is clear insofar as the internal properties of its sense are shown not only by internal properties of the proposition but also by internal properties of the propositional sign.
(quoted from Ramsey's critical notice in "The foundations of mathematics and other logical essays").
What is the difference between a "proposition" and a "propositional sign"?
See Frank Plumpton Ramsey, The Foundations of mathematics (1931), page 274:
A propositional sign is a sentence ; but this statement must be qualified, for by 'sentence' may be meant something of the same nature as the words of which it is composed. But a propositional sign differs essentially from a word because it is not an object or class of objects, but a fact, "the fact that its elements, the words, are combined in it in a definite way " (3.14). Thus 'propositional sign' has type-token ambiguity ; the tokens (like those of any sign) are grouped into types by physical similarity (and by conventions associating certain noises with certain shapes) just as are the instances of a word. But a proposition is a type whose instances consist of all propositional sign tokens which have in common, not a certain appearance, but a certain sense.
Consider this example:
"Napoleon is the Emperor of France"
and
"Napoleon is the Emperor of France";
they are two different tokens of the same type, because they are "individuals" differing by font (as well as for spatial coordinates), like two different utterances of the same sentence.
They are instance" of the same propositional sign, which is a linguistic object.
The sentence:
"The Emperor of France is Napoleon"
is a different type of sentence, i.e. a different propositional sign.
Of course, all them have the same meaning, i.e. they express the same thought or content: they are related to the same fact of the world.
This fact (regarding Napoleon and France) is the proposition, which is not a linguistic object.
In the Tractatus, a propositional sign is not the sentence type, as Allegranza suggests. In fact, it need not be a sentence token, either: It is whatever particular fact that makes a proposition available to the senses. So, it might be an arrangement of audible sounds, visible ink on a piece of paper, etc. (Wittgenstein stretches the notion of picture, or proposition, very far, so accordingly the notion of propositional sign is very wide.) Of course, the propositional sign is not an event, e.g. your hearing the sounds, or your seeing the ink, but the particular configuration of elements that flow through the air (in the case of sound) or that is marked on the paper (in the case of ink).
So, "p" and "p" are two different propositional signs.
Likewise, Wittgenstein's conception of the proposition is not the Russellian one. What a proposition shows, to anyone who understands it, is the possible fact that would make it true. Whether or not this possible fact is a fact 'of the world', i.e. exists, depends on whether or not the proposition is true. The proposition says that this fact actually exists.
So, there is no question of how the proposition can 'exist' if it is false, since the proposition is independent of the fact that makes it true.
The proposition itself is also a fact. Propositions are logical pictures or models of reality which we gather from the propositional sign by analyzing its internal structure, in order to arrive at a logical form, a structure of constituent signs. The logical form projects a situation independently of form of representation (i.e. the form that the propositional sign/picture takes, including whether it be an arrangement of colour, sound, shapes, etc.), because we look for that very same logical form in the possible fact (in the world).
So the proposition is not really a different fact from the propositional sign, but is just the "pictorial sign in its projective relation to the world". It is, as it were, the propositional sign considered as a model, rather than just a bunch of sounds or marks.
In the Tractatus Wittgenstein adheres to the ontology of Russell's "logical atomism", where "in the analysis of propositions we must come to elementary propositions, which consist of names in immediate combination” (TLP 4.112), and elementary propositions express irreducible atomic facts. Both Russell and Wittgenstein were weary of "shadowy entities" expressing the content of propositions, like senses or meanings, so proposition is just a “propositional sign in its projective relation to the world” (TLP 3.12), i.e. to the atomic facts upon which its elementary constituents rest.
Ramsey's quote most likely refers to Wittgenstein's contention that in the right kind of language "Logic must take care of itself... In a certain sense, we cannot make mistakes in logic" (TLP 5.473). One of the points of the Tractatus is to introduce the right kind of language, Wittgenstein's concept-script, translation into which would eliminate meaningless sentences automatically because it is structured to not be able to express them. In the ordinary language many signs do not "say" what they mean, which is why we often miss when they mean nothing at all, and succumb to conceptual confusion. In the ideal language that the concept-script strives to be, the clarity of meaning is to be reflected directly in the clarity of notation: propositions either transparently reduce to empirical atomic facts, or reveal their meaninglessness. See Development of Wittgenstein's Philosophy by Kuusela
