The second phrase is not an independent clause, which is the issue. > the only punctuation mark that regularly unites readers and writers in deep-seated repugn

The second phrase is not an independent clause, which is the issue. > the only punctuation mark that regularly unites readers and writers in deep-seated repugnance There is not a subject and a verb; this is a fragment and does not stand alone. Prepending "it is" to the second clause would make this a proper sentence by making the second clause an independent clause instead of merely a noun modified by an adjectival prepositional phrase. Edit: I nerdsniped myself and it might not be strictly accurate to call it a prepositional phrase; 'that' is technically a conjunction in this case used to link its own subordinate clause, but that subordinate clause still does not complete the clause fragment whose subject is "[punctuation] mark." It's still just a modified noun.