Links shown in green are those believed to be in operation during the “core” period of the BT (PO) network – the 1960s to 2005 or beyond. Links shown in grey are early or temporary links which closed by the end of the 1960s, normally due to network development.

Links added to existing sites during the 1980s are shown in orange to highlight the additional routes. Sites added in the 1980s, however, have their links shown in green unless there is a reason to show progression. Occasionally orange has been used to show links added in the 1970s where a distinction is useful (for example Quernmore).

Cable links between adjacent sites and experimental links between sites which bypassed the normal routing are shown in red – please refer to the text on the page for more details.

Where sites are in close proximity to each other but were not interconnected (for example Blackford Hill and Craiglockhart, Edinburgh) a yellow marker has been added. This has also been done in certain cases where two routes cross close to each other but are not connected (for example Pimlico and Harrow Weald).

As far as possible links have been sequenced to allow easiest progression along the route (north/south or west/east). For sites with more than two links the sequencing is normally clockwise from west.