It wasn't until the 19th century that infant beds developed from bassinettes, acquiring a role of keeping the child in their bed. The development of a distinction between infant beds and bassinettes was natural because it was "considered vital that the child's bed be raised off the ground." This was due to a perception of noxious fumes below knee level, and explosive vapours near the ceiling, with good air in between. Once children's beds were raised off the ground the role of the sides changed from a convenience to a safety feature.