sentences:  London postal arrangements were refined in 1917 when all its postcode districts (seven radial which are large and two innermost, much smaller) became publicly sub-divided; these were named after the most important parish, chapelry, natural topological or built feature at that time in that area on which their numbering rests.As London is one post town, district names are deprecated, in favour of the post town LONDON to be written/typed.Within each NW postcode district, PO boxes are allocated to a unique postcode sector, except for two districts which use all available sectors for ordinary addresses and therefore have their separate non-geographic districts