The X A (Y) D comparative contains a covert weak comparative morpheme (i.e. the covert verbal suffix -guo2) that is grammaticalized from the verbal suffix -guo1, meaning 'exceed' or 'surpass', in the X A-guo1 Y (D) comparative. The ”weak” nature of -guo2 is the ”virus” that makes the X A (Y) D comparative differ from the X bi Y A D comparative not only in obligatoriness of the measure phrase but also in further modification by degree adverbs. And most of the properties specific to the X A (Y) D comparative in fact originate from the weak nature of the covert verbal suffix -guo2.