London's squares are arranged by postcode, see the map below of postcodes. Like British, Irish and Dutch postcodes, Canada's postal codes are alphanumeric. Radio callsigns pre-date postcodes in Australia by more than forty years. The codes are also used for other purposes, such as statistics and postcodes. At the same time, ordinary "geographical" postcodes replaced the SO9 codes. There are 1856 postcodes, each of which may serve up to 10000 individual locations. Spatial representation of postcodes is also very popular in defining sales and franchise territories. The square is the largest of the two centremost postcodes (prefixed WC and EC). Bigger cities were formerly divided into districts, which often lives on in postcodes. Addresses receiving large volumes of mail are each assigned separate "large user" postcodes. They were to be replaced by house numbers, road names and postcodes. The roads surrounding the common fall within the SW4, SW11 and SW12 postcodes. However, Royal Mail sometimes assigns semi-mnemonic postcodes to high-profile organisations. However, postcodes with the same second number are not always next to each other. On 1 May 1967 postcodes were introduced in Croydon.