This county constituency until 1832 covered all the historic county of Middlesex, in south-eastern England, comprising Spelthorne, Poyle, South Mimms and Potters Bar in other modern counties, together with the north, west, and north-west sectors of the present-day Greater London. Apart from the ability of some voters to participate in the borough franchises of the cities of London and Westminster (after dates of their inception, see top right or below), it gave rise to three more urban offshoot divisions in 1832, one of which was split in two at the next national review or reform, in 1868. Its southern boundary was the River Thames.