A cul-de-sac in Whitehall, once (before Mrs T.) just like any other (ie you could walk up and down it) but now closed to public access. North side of the street contains the official residences of the Treasury Ministers - the Prime Minister (at [Number 10]?) and the Chancellor (at Number 11). The Chief Whip's offices (officially the Parliamentary Secretary to the Treasury) were at Number 12, but this is now part of the Number 10 office. So Number 9 was invented - actually just a door on to the [Cabinet Office]?.
The south side of the street is bounded by the [Foreign Office]?.