A party wall (occasionally parti-wall or parting wall, also known as common wall) is a dividing partition between two adjoining buildings (or units) that is shared by the tenants of each residence or business. When built for this purpose, the builder will lay the wall along a property line dividing two terraced flats or row houses, so that one half of the wall's thickness lies on each property. This type of wall is usually structural. Party walls can also be formed by two abutting walls built at different times. The term can be also used to describe a division between separate units within a multi-unit apartment complex. Very often the wall in this case is non-structural but designed to meet established criteria for sound and/or fire protection between residential units.