The Mortal Instruments
“Mortal instrument” is a legal term, referring to an implement capable of killing someone. For example, from the records of the Cape Colony (circa 1793):
It was decided by our law, in the event of persons having assaulted others
actually with a mortal instrument, and wounded them in such a
manner that death was the consequence; if it only appears that they
had an intention to wound, not to kill, these have escaped the
punishment of death.
Another possible source is Shakespeare, specifically Julius Caesar:
Since Cassius first did whet me against Caesar,
I have not slept.
Between the acting of a dreadful thing
And the first motion, all the interim is
Like a phantasma, or a hideous dream:
The Genius and the mortal instruments
Are then in council; and the state of man,
Like to a little kingdom, suffers then
The nature of an insurrection.
Here the “mortal instruments” are Brutus’s emotions or perhaps his body itself, which revolt against the idea of killing Caesar.
It seems likely that Cassandra Clare was familiar with both meanings.
In-universe, of course, the term refers to the three artifacts that the angel Raziel granted to the Shadowhunters: the Mortal Cup, the Mortal Sword, and the Mortal Mirror.
The meaning of most of the series titles is a bit unclear. City of Glass certainly refers to Idris, the city of the Shadowhunters.
I’m going to Idris! her mind sang. I’ll see the Shadowhunter home
country, the City of Glass. I’ll save my mother.
City of Glass
On the other hand, most of the other titles take place in (say) New York, and so probably are not referring to different cities. The part after City of seems to refer to elements important to the book’s plot, though. For example, heavenly fire is rather important in the book of the same name.
The Infernal Devices
In keeping with the legal theme, an “infernal device” or more commonly, “infernal machine,” is a rarely-used term for an incendiary device, i.e. a bomb. From a Massachusetts law:
For the purposes of this section, the term hoax device shall mean
any device that would cause a person reasonably to believe that such
device is an infernal machine. For the purposes of this section, the
term infernal machine shall mean any device for endangering life or
doing unusual damage to property, or both, by fire or explosion,
whether or not contrived to ignite or explode automatically.
For the specific term “infernal device,” see for example here.
The term might refer more broadly to any device that operates by fire, i.e infernally.
Of course, more colloquially “infernal device” is an very old-fashioned way of saying a “wicked plot or scheme.”
Finally, it is worth noting that there is a book called Infernal Devices, published in 1987, which (most tellingly) is a steampunk novel that features a clockwork person, rather like Clare’s series of the same name.
In-universe, the term refers to the demon-possessed clockworks created by Axel Mortmain, which fit the term triply: they are (a) clockworks like those in the 1987 novel mentioned previous, (b) may run on steam or some similar fiery internal mechanism, and (c) are infernal in the sense of being possessed by entites from Hell.
As for the book names, they are quite straightforward:
The Clockwork Angel is a literal device that protects Tessa.
The Clockwork Prince is Mortmain.
“He’s not frightened at all, Anne.” The man laughed, and set the boy
down on the ground, ruffling his hair. “My little clockwork prince . .
The Clockwork Princess is Tessa herself.
The Dark Artifices
The title comes from Tacitus, according to an interview with the author:
The inspiration for the title! Okay, this is so dorky. The main
family, the Blackthorns, of “The Dark Artifices,” have a father who is
a classicist. He is obsessed with Greek and Roman mythology and
history. So “The Dark Artifices” comes from “The Annals of Imperial
Rome” by Tacitus. “It would have been less ignominious to die by the
dark artifices of Tiberius or the fury of Caligula.” I just liked the
As for the titles of the individual books:
Lady Midnight comes from a song by Leonard Cohen:
I cried, “Oh, Lady Midnight, I fear that you grow old, the stars eat
your body and the wind makes you cold.”
In-book, it refers to a character from a Shadowhunter tale:
“Great-Aunt Marjorie gave it to me,” Tavvy said. “I like most of the
stories. The one about the first parabatai is good, but some of them
are sad and scary, like the one about Tobias Herondale. And the one
about Lady Midnight is the saddest.”
“Lady what?” said Cristina, leaning forward.
“Midnight,” said Tavvy. “Like the theater you went to. I heard Mark
say the rhyme and I just remembered I read it before.”
Lord of Shadows may be a reference to Satan, who is often called the Prince of Darkness. It might also be a reference to the Wiccan God (as in God and Goddess, or Lord and Lady), who is sometimes referred to as the Lord of Shadows.
Clare has confirmed that in-universe, the Lord of Shadows is in fact a real person:
Yes, the Lord of Shadows is a real person.
As previously mentioned, Queen of Air and Darkness is probably a reference to Queen Mab, who is referred to by that name in various previous works of fiction, notably Dungeons and Dragons. That said, the term apparently originally referred to Morgause from The Once and Future King.
Whatever the explanation may have been, the Queen of Air and Darkness
had a baby by her half-brother nine months later. It was called
The Last Hours
The series has a heavy dose of Dickens influence, so it is no surprise that, as noted by this site, the title appears to come from Great Expectations:
The stones of which the strongest London buildings are made, are not
more real, or more impossible to be displaced by your hands, than your
presence and influence have been to me, there and everywhere, and will
be. Estella, to the last hours of my life, you cannot choose but
remain part of my character, part of the little good in me, part of
The titles of the books (Chain of Thorns, Chain of Gold, and Chain of Iron) also come from Great Expectations:
Pause you who read this, and think for a moment of the long chain of
iron or gold, of thorns or flowers, that would never have bound you,
but for the formation of the first link on one memorable day.
The Wicked Powers
This series is quite a ways off, so it’s hard to tell. Given that Clare seems to like Shakespeare, though, it could be a reference to The Winter’s Tale:
Quit presently the chapel, or resolve you
For more amazement. If you can behold it,
I’ll make the statue move indeed, descend
And take you by the hand; but then you’ll think–
Which I protest against–I am assisted
By wicked powers.
The titles of the individual books are not yet known.