has requested the narrator's help. It turns out that Roderick's sister,Madeline, is also ill. She has a sickness that puts her into a cataleptic, deathlike state, and doctors predict that she will soon succumb to it.

Madeline abruptly dies, and Roderick decides to preserve her corpse in a dark vault within the house for two weeks before burying it. The narrator assists Roderick in carrying the coffin to the vault, which is small, damp, and enclosed behind a door of heavy iron. After setting down the coffin, they partially open the lid to look inside. This is when the narrator makes the comment:

A striking similitude between the brother and sister now first arrested my attention; and Usher, divining, perhaps, my thoughts, murmured out some few words from which I learned that the deceased and himself had been twins, and that sympathies of a scarcely intelligible nature had always existed between them.

According to the Merriam-Webster Dictionary, the word "similitude" means "counterpart, double; a visible likeness." We can see that this definition applies in this case. In using the word "similitude," the narrator is remarking on the similarity in appearance between Roderick and Madeline. This is confirmed when Roderick confesses that he and his sister are in fact twins.