Home threshold

Homes are places of special magic qualities, which are surrounded by and separated from the outside by a threshold.

Its most prominent effect is its barrier-like quality to magic, making crossing the threshold impossible or inconveniencing to magic and 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 strenght, and not every house qualifies as a home. The existence and strenght of a threshold depends on the way it is inhabited. Places that feel like a home to its inhabitants are stonger than those that are simply a place to live, which are "just a house". Familiy 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.

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.

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.

Humans, as a non-magical creature, 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.

Supernatural and magic creatures 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.

In the series
Harry Dresden's apartment does not have a strong threshold, as it is rented and a bachelor single household. Karrin Murphy's house has a strong threshold, as the house has been a home for a century and to generations of her familiy.

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.