In propositional calculus especially, there is a very small number of symbols available, and a very small number of ways they can combine. So it's tempting to think of this small stock of mostly special characters ( ¬ ⋀ ⋁ ⊃ ≡ p q r s ) as an alphabet.
In fact, however, these letters don't represent parts of words -- rather, they are words, and thus represent parts of sentences. For instance, ¬ means not, ⋀ means and, and actual alphabetic letters like p, q, r, s are used to represent any proposition (i.e, sentence) at all. In other words, logic represents Sentences, and therefore its parts represent Words. That's the Word/Sentence metaphor.
If, however, one refers to the stock of symbols as an alphabet, then they are Letters and thus they make up Words, not sentences. This is the Letter/Word metaphor. They're not incompatible, but they do require one to switch assumptions, and to treat logic as mathematics (which uses formulae and letters) instead of semantics (which uses words and sentences).
For details, see my Logic Study Guide