The Three Musketeers by Alexandre Dumas Chapter 10 Page 21

“But that will compromise me sadly, you know.”

“Of what consequence? Nobody knows you. Besides, we are in a situation to overlook ceremony.”

“Come, then, let us go to your friend’s house. Where does he live?”

“Rue Ferou, two steps from here.”

“Let us go!”

Both resumed their way. As d’Artagnan had foreseen, Athos was not within. He took the key, which was customarily given him as one of the family, ascended the stairs, and introduced Mme. Bonacieux into the little apartment of which we have given a description.

“You are at home,” said he. “Remain here, fasten the door inside, and open it to nobody unless you hear three taps like this;”