What narrative technique does Dickens employ in Chapter X?Book the Third of A Tale of Two Cities by Charles Dickens

In Chapter X of Book the Third of A Tale of Two
Cities, Charles Dickens employs the literary technique of Back-story with the
letter of Dr. Manette, formerly prisoner One Hundred and Five North Tower of the
Bastille, that is discovered by Ernest Defarge upon the revolutionaries storming of the
old political prison. This letter of Manette's provides background information that is
most significant both to character and to events of the
plot. 

The letter is one that Dr. Manette has written in
his tenth year of incarceration, using the rusty iron point of a nail that he scrapes
into soot and charcoal.  Manette explains the history of his being imprisoned after
having been taken by the Evremonde twin brothers who were armed.  In a type of barn,
Manette finds both a dying young woman and a boy of seventeen.  One of the Evremonde
twins have slain him and caused the woman her death.  After they both die, the brothers
offer Manette money, but he refuses it.  On the next day after his return home, Dr.
Manette finds a bag of gold outside his door; nevertheless, he pursues his intention to
write to privately to the Minister of the past affair.  Before he sends his letter,
however, Manette has a visitor, the wife of one of the brothers Evremonde.  She worries
that God will punish her son for the sins of his father; in her hopes of averting this
punishment she seeks the living sister.  But, Manette cannot help her.  So, he sends his
letter.  On that night, he is tricked into thinking he is called to aid someone ill,
but, instead, is captured and taken to prison, the prison from which he records his
history of the fateful incidents.

Later in the novel, the
testimony of this letter is crucial to incidents involving Madame DeFarge and Dr.
Manette and Charles Darnay.  Dr. Manette's letter as back-story is also central to
clearing up the identities and occurrences involving Madame Defarge and
Darnay.