Threshold

Thresholds are entities separating homes from the outside. They most prominently constitute a barrier to magic, making crossing them impossible or inconvenient to magical beings unless invited in.

Description
A threshold is a magical barrier around a home; it acts like an invisible and intangible field of energy surrounding it and keeps out unwanted magical forces. Crossing a threshold may be impossible or come with great effort or at a cost for some beings unless invited in.

Every home is surrounded by a threshold, although there are great variations in strength, and not every living space qualifies as a home. The existence and strength of a threshold depends on the way it is inhabited. Places that feel like a home to its inhabitants, that are connected to emotions and have a history, are stronger than those that are simply a place to live, which are "just a house". Family homes are generally stronger than single households. Old and longtime dwellings, ancestral homes, places with a history of their own are usually very strong, whereas short-term or rented living spaces are weaker.

Details

 * Places that are not homes, like public buildings, hotel rooms, offices, and so forth, as a rule do not have a threshold.


 * The magic qualities of a home and its threshold exist only around homes of mortals (possibly only humans). Dwellings of other beings do not induce the existence of a threshold.
 * Monsters do not get a Threshold.

Crossing a threshold
Crossing a threshold is a meaningful action, in which both the magic barrier and the crossing person influence the situation.

If invited into the home by its owner or an inhabitant, the invited person or being can pass into the home completely unaffected. Trying to cross a threshold uninvited has repercussions, or may not be possible at all. Crossing a threshold uninvited leaves much of the crossing being's magic behind, such as a Wizard.

Dresden has described the feeling of a threshold as a kind of tension, when crossing uninvited as something tugging at him and having to "push" through , feeling a resistance, and experiencing a tingling sensation.

Effects on Various Groups
Humans, are the main group of beings who actually get a threshold. As a non-magical creature, humans can cross thresholds without invitation or harm. Normal humans possessing no magic as a rule can and do pass thresholds without noticing or being affected; although they themselves can and do influence it, as lots of strangers entering a home can weaken the threshold (at least temporarily, and possibly only when uninvited).

Human wizards are able to cross a threshold uninvited, although forcing themselves through the barrier leaves much of their magic and power behind, severely weakening their magic while inside the home. If wizards are invited in, they can pass without loss of or other influence on their magic.

Vampires:
 * Red Court: crossing a threshold would render them nearly paralyzed.
 * Black Court: can not cross a threshold at all.
 * White Court: can cross a threshold but it cripples their ability to draw on their Hunger for strength and speed.

Faeries: Beings from Faerie don't need necessarily need to be invited over a threshold if they are benevolent to the inhabitants of the house. While there, they are bound by the Laws of hospitality—the obligations of guest and host.

Supernatural and magic creatures, including demons,  are much more affected and inconvenienced by a threshold, due to being comprised of and bound to a greater extend by magic and its rules. Forcing themselves through a threshold would not only result in the loss of their 'outer' magical powers while in the home, but often actively endanger their life. Thus many magical creatures are often incapable of crossing a threshold, due to being kept out completely by the barrier, or due to unwillingness to endanger themselves to this extend.

Even if a creature tries to or can force itself through a (possibly weaker) threshold, it usually still takes a lot of effort. All this results in many, if not most magical creatures being unable to cross a threshold unless invited in. Among them are for example demons, or vampires.

Examples of thresholds
Harry Dresden's apartment does not have a strong threshold, as Harry rents it, lives alone, and has only been in it for about ten years.

Karrin Murphy's house has a strong threshold, as the house has been a home for a century and to generations of her family.

In the series
In Storm Front, when Harry Dresden's apartment is invaded by a Toad Demon, Kalshazzak, it stalls at the door for a while. Harry tells Susan Rodriguez that it's because of "Homestead Laws". It eventually pushes its way through Harry's threshold.

In Death Masks, when Susan returns, Harry makes her cross his threshold to test whether she's still Susan or if she has been fully changed into a Red Court vampire.

In Dead Beat, Harry explains to Waldo Butters what a threshold is and how Murphy's home has a strong one. A home has to have a history, family, emotions worked into the wood. The Murphy family had that home for better than a hundred years and lived in for every one of them. The threshold is solid and would keep out the Erlking.

In White Night, Harry crosses Anna Ash's threshold twice without an invitation. The first time is when he foes to a meeting of Ordo Lebes to talk to the women. Anna greets him with suspicion and forces him to enter without an official invitation purposely to strip his magic. The second time is when Harry and Mouse return for more questions. Harry senses a sleeping spell in the building and hears a suspicious sound, thinking Anna in trouble, Harry breaks Anna's ward and once again enters sans invite and again his magic is stripped.

In Ghost Story, the White Court vampire—Felicia—crossed Murphy's threshold without an invitation. Harry said that was gutsy because it cripples the White's ability to draw on their Hunger for strength and speed.

In Cold Days, Harry summons Cat Sith into Thomas Raith's apartment. Thomas is quite disturbed that a being from Faerie can just pop right in without being invited. Harry and Cat Sith assure him that he is obligated to abide by the Laws of hospitality, which is near holy to Faeries.