"I know your true name, Natty boy. That means I have some power over you. It's not all one way anymore, is it?"
Nathaniel, Eh

Nathaniel denies his true name to Bartimaeus.

The true name of an individual was an important aspect of magic. For humans, this was also known as the birth name.

For demons, the true name allowed a magician to summon them. Any magician possessed of the proper knowledge and ability could summon a demon if they had its true name, provided that the demon in question was not currently in the service of another magician and was still extant. As such, magicians and demons both often zealously guarded knowledge of such names, in order to prevent being summoned, or to prevent summoning by another magician. In some societies, the practice of bynames was used, in which a demon was known by a different name than its true name. Such names were often re-used, for example, Rekhyt, a byname of Bartimaeus, was also used by at least two other demons of note. True names of demons were often recorded in books for use by future magicians.

For magicians, a true name could be a source of great danger. Many magicians, for their own safety, did not even know their true names. Those that did generally consigned them to oblivion upon coming of age, choosing to hide behind an assumed name by which they were known to the world. Knowledge even of this assumed name was useful to demons, but gaining knowledge of the magician's true name was considered a source of great power. It provided the demon with a key defense by allowing the demon to turn any punishment spell cast by the magician against them, simply by stating the magician's true name, though it was still possible for the magician to dodge. In theory. this provided the demon with supreme power, but there were limitations that could be exploited by a clever magician. The demon was still technically under the power of the magician's summons and compelled to obey their orders. Additionally, knowledge of the true name still did not allow the demon to avoid certain tricks such as the spell of indefinite confinement, which was essentially a dismissal, but trapped the demon in a prison on Earth. This loophole was used the boy Nathaniel, later known as the magician John Mandrake, when his true name was learned by Bartimaeus.