Spear closet

A spear closet is an architectural “left over” space of small size and irregular shape.

When laying out floor plans for buildings, designers sometimes are unable to avoid small “left over” spaces which cannot be put to practical use. This problem is particularly severe in buildings with angular or curvilinear geometry. These spaces are called “spear closets” because, due to their size and shape, they cannot serve any practical purpose other than possibly to store one’s spears.

Because very few people in the modern world own spears and keep them at home, the term is ironic. However, it is a slang term in use among architects in the English-speaking world. A spear closet may have a door and be used as a storage closet. However, it may also simply be a “dead” space in a building which is completely surrounded by walls and which is inaccessible.

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Wikipedia