Pairing is such an essential part of this excellent novel, and if we analyse it carefully we can see how many different aspects of the novel have been paired together. Of course, one of the most prominent pairings is that of the two characters of Sydney Carton and Charles Darnay. The pairing between them is built around their similar physical appearance, and becomes crucial for the denouement of the novel. The pairing is first introduced in Chapter Three of Book the Second, when Stryver manages to secure the acquital of Charles Darnay, charged with treason, based on his similarity to Sydney Carton. Note how the novel introduces this similarity:
Allowing for my learned friend's appearance being careless and slovenly if not debauched, they were sufficiently like each other to surprise, not only the witness, but everybody present, when they were thus brought into comparison. My Lord being prayed to bid my learned friend lay aside his wig, and giving no very gracious consent, the likeness became much more remarkable.
Of course, as we discover, the similarity is only skin deep, as the two characters possess very different personalities. However, it is this similarity that creates a bond between them, and also gives Sydney Carton the opportunity he has been looking for to redeem himself.