From On Her Majesty's Arcane Service
Fairies are creatures closely related to humans, but different enough so that they hardly ever interbreed successfully. Fairies preferentially live in pockets, either natural or made by a Savant, as they can’t make their own. Some fairy pockets are tiny, and others are whole universes. They generally have guarded entrances held permanently open by rocks, timber, or other props.
Faerie is a realm co‐existant with the human realms, hidden away in pockets of extra-dimensional space and complete pocket universes. In the British Isles ‐ England and Wales, Scotland, Ireland, and the smaller islands ‐ there are hundreds of these pockets, ranging in size from tiny croftholds with less than five inhabitants to Faerie cities with thousands of inhabitants. Each Faerie pocket has different base rules, different physics, than human realms; and different from each other. Time may be faster or ‐ usually ‐ slower. Distances inside the pocket can be enormous even though the pocket may be tiny externally. Magic can be performed casually, and cause and effect are different, perhaps reversible.
The people of these realms are Fairies. Most larger fairy pockets are independent kingdoms. Fairie pockets generally have different timescales than our world ‐ fairies are conservative and don’t like change, so they like their pocket time to run slower than the real world. Sometimes human get into fairy pockets, either invited in or as Changelings, and sometimes hundreds of years might pass in the outside world before they return. Not all fairies live in pockets, however. Some have left their pockets to wander, some have been exiled from their communities, some prefer to live in the real world, and some simply have no home pocket to live in. Such fairies
occasionally commission Savants to create new pockets for them.
Fairy children are much like human children, they are the same size and shape as human babies, and left to themselves will grow up to look and act very human indeed. In the fairy societies, though, fairies grow up to become outwardly what they are inwardly. Murderous fairies become Red Caps. Brutish fairies become Ogres and Giants. Innocent, flighty fairies become Piskies. Fairy communities are almost always of one type. A fairy born into a community of Piskies, for example, who shows signs of becoming a Red Cap will be forced out of the community to wander until he can find a Red Cap community to join.