In the United States, the type of rooms described by the term den varies considerably by region. It is used to describe many different kinds of bonus rooms, including studies, family rooms, home offices, libraries, home cinemas, or even spare bedrooms.
In some places, particularly in parts of the British Isles, a small den may be known as a snug.
While living rooms tend to be used for entertaining company on formal occasions, dens, like other family rooms, tend toward the more informal. In houses that do not have dedicated family rooms or recreation rooms, a den may fill that niche. Dens can also be private areas primarily used by adult members of the household, possibly restricting access to the room by their children. Dens with home theater systems and large screen televisions may be referred to as media rooms instead. Most den floors are made out of wood, carpet, or floor tiling.
In Canada, the word "den" is frequently used in real estate listings (in accordance to Canadian building codes) to refer to a room in a condo without a window. 
- "den". New Oxford American Dictionary (3rd ed.). Oxford University Press. 2010. ISBN 9780199891535.
a small, comfortable room in a house where a person can pursue an activity in private
- Media related to Dens at Wikimedia Commons
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