The Scottish Parliament constituencies have not been coterminous with Scottish Westminster constituencies since the 2005 general election, when the 72 former UK Parliament constituencies were replaced with a new set of 59, generally larger, constituencies (see Scottish Parliament (Constituencies) Act 2004). The size difference between Westminster and Holyrood boundaries was due to diverge further upon the implementation of the Sixth Periodic Review of Westminster constituencies, which has not been voted upon by Parliament. The 2023 Periodic Review of Westminster constituencies for a UK total of 650 MPs will commence in 2021.