Maintenance of the HDB's approximately 900,000 units largely falls under the Town Councils, which are not part of the HDB but which are formed under the Town Councils Act primarily with the purpose of maintaining the common areas of HDB flats and estates. Common areas would include the common corridors, void decks, lifts, water tanks, external lighting and the open spaces surrounding the estates, which are managed, maintained and improved on by the respective Town Councils. These Town Councils are formed by the respective political constituencies and do not necessarily follow HDB Town boundaries, hence a single HDB Town may be managed by multiple Town Councils.