4.012 It is obvious that a proposition of the form 'aRb' strikes us as a picture. In this case the sign is obviously a likeness of what is signified.

There is nothing obvious about it, and indeed I think it is false. My reason is not that letters and sequences of letters are not drawings, for they are, but that pictures - leaving so-called abstract art aside - visually are evidently like what they are pictures of. By contrast 'aRb' is a schematic piece of algebraic notation, and to say that it "strikes us as a picture" is as sensible as saying that "a+b" in group theory "strikes us as a picture", provided the group is not commutative.

What W. confuses here is the fairly accidental fact that 'aRb' is used in the Principia Mathematic for a binary relation R between a and b, and the not so accidental fact that written language is composed of ordered sequences, which can be represented by binary relations.