The general purpose of glamour is to hide the Shadow World from mundanes. It makes things look different from how they truly are. Performed correctly, it creates a perfect semblance in the mind of the observer and perfectly obscures the true shape of the thing glamoured.
Having what is referred to as "the Sight" enables one to see past glamours to a thing's true nature. Most Shadowhunters and some mundanes are born with the Sight, and it also comes naturally to Downworlders. However, this does not guarantee that they can see through all glamours. For some, seeing through basic glamour may still require some training or experience. In other cases, glamour may be reinforced and more powerful than typical glamour.
Glamours may put a "false skin" over something, disguising whatever it is placed on, such as Institutes, which is sometimes glamoured to blend in with the surroundings. Nephilim also often glamour themselves into invisibility, to move undetected through the mundane world. Similarly, demons may glamour themselves into nonspecific forms, so that a mundane attacked by a demon will perceive it as something generic, like a dog or another random mundane.
The glamour is considered to be one of the simplest and most widely and extensively used magic in existence. It is one of the few kinds of magic available to all magic users—it is found in the Gray Book, in demonic spell books, in the researches of warlocks, and among the fey.
The origin of glamour magic is a subject that has puzzled generations of magical researchers. There have been various theories, the most common of which historically has been that glamour magic originally belonged to the fey and was "stolen" from them by other creatures somehow.
Most glamours are easy to see through for any magical being, and they usually hide things only from mundanes. Vampires, faeries, and warlocks all may use more powerful glamours to hide their activities not just from mundanes but also from Shadowhunters and from one another. Faeries especially are considered the masters of glamour magic.
The unusual features of Downworlders, particularly of warlocks and ifrits, can be glamoured into invisibility. And since ifrits are incapable of doing magic to hide their marks, they usually acquire magical artifacts that provide them with a permanent glamour and live away from the magical world.
Another prevailing theory among the Nephilim is that there are at root two types of glamours—the angelic glamours that Nephilim use, produced by the inscription of Marks, and demonic glamours, used by everyone else. It is assumed that Raziel granted Shadowhunters the power of glamours and the power of Sight.
The Voyance rune helps Shadowhunters focus their Sight. In some cases though, they need to be trained to see through glamours. Usually, it is sufficient to deliberately show children glamours of various kinds and their revealed true shapes; however, more complicated glamours are more difficult to see through.
Shadowhunters use glamours to their advantage when fighting demons, which are apparently unable to perceive the difference between mundanes and Shadowhunters, and unable to sense the working of angelic magic. They also perform glamours in situations where the Shadow World is in danger of discovery by the mundane population. They often glamour themselves into invisibility, to move undetected through the mundane world.
The Sight is the only means of seeing through glamours; just knowing that something has been glamoured, or even knowing its true shape, does not remove the glamour effect. This is notable because there are many folk beliefs among mundane cultures about rituals and tools that can be used to see through glamours. Some of these may successfully work to help see through faerie glamours, by means not completely understood. Tools such as clary sage, Seeing Stones, wearing clothing inside out, washing your face in a particular spring at sunrise, and so on—cannot, however, be used to see through the more common glamours used by warlocks, demons, and Shadowhunters.
Most mundanes with the Sight, however, still never notice any supernatural activity, because they are not prepared to see it. An important rule of glamours is that, for the most part, people see what they wish to see, and things that they can make sense of. Sighted mundanes will often look past strange appearances and explain them as illusions or misunderstandings.