Also common especially in older flats are the common corridors, some of which may run across the length of slab blocks. Considered public property, they have rules preventing home owners from occupying and restricting movement, with the exception of units at far ends of corridors who may purchase and incorporate parts of the corridor into their units from the HDB. While these corridors are welcome for being the default interaction areas for neighbours and their children, and the added sense of security due to their open-nature, issues of privacy can crop up, resulting in more contemporary blocks featuring far less units per corridor. Larger units such as 5-room flats are also commonly housed in "Point blocks", which feature only four units per floor. Early HDB blocks tend to be of a single standard slab design of uniform height, typically averaging 12 storeys and arranged equidistant from each other. — Wikipedia (probably from HDB)