The fact that the propositions of logic are tautologies shows the formal - logical - properties of language, of the world.
That its constituent parts connected together in this way give a tautology characterizes the logic of its constituent parts. In order that propositions connected together in a definite way may give a tautology they must have definite properties of structure. That they give a tautology when so connected shows therefore that they possess these properties of structure.
6.1201 That e.g. the propositions "p" and "~p" in the connexion "~p . ~p" give a tautology shows that they contradict one another. That the propositions "pq", "p" and "q" connected together in the form forma "(pq). (p)::(q)" give a tautology shows that q follows from p and pq. That "(x).fx::fa" is a tautology shows that fa follows from (x) . fx, etc. etc.
6.1202 It is clear that we could have used for this purpose contradictions instead of tautologies.
6.1203 In order to recognize a tautology as such, we can, in cases in which no sign of generality occurs in the tautology, make use of the following intuitive method: I write instead of "p", "q", "r, etc., "TpF", "TqF", "TrF", etc. The truth-combinations I express by brackets, e.g.: