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Reality Checkpoint is the name given to a large lamp-post in the middle of Parker's Piece, located at the intersection of the park's diagonal paths. The name comes from an unofficial inscription which has been painted on the lamp-post since the early 1970s. The lamp-post is also believed to be the oldest electrical lamp-post in Cambridge.
There are four main theories as to the meaning of the name.
1.It may mark the boundary between the central university area of Cambridge (referred to as the 'reality bubble') and the 'real world' of non-academic locals living beyond. One is warned to check one's notions of reality before passing. For students at Cambridge, who walk out to Mill Road across Parker's piece for an evening in the 'real world', usually including a visit to one of Mill Road's selection of pubs, the lamp-post marks the end of the 'reality holiday' as they walk back to central Cambridge - back into 'the bubble'.
2.The name arose because the lamp-post forms a useful landmark for people crossing the park at night—perhaps inebriated or in the fog—since it is the only light for hundreds of yards.
3.When drunk, students and the general public are reminded to check they are able to walk like a sober person before passing the police station just a few hundred metres away.
4.The post being situated in the middle of two walking paths that intersect, anyone walking whilst not tuned in to "reality" will probably collide with the lamp-post, hence "reality checkpoint".

Listed buildings and structures are officially designated as being of special architectural, historical or cultural significance. There are over half a million listed structures in the United Kingdom, covered by around 375,000 listings.
Listed status is more commonly associated with buildings or groups of buildings, however it can cover many other structures, including bridges, headstones, steps, ponds, monuments, walls, phone boxes, wrecks, parks, and heritage sites, and in more recent times a road crossing (Abbey Road) and graffiti art (Banksy 'Spy-booth') have been included.

In England and Wales there are three main listing designations;
Grade I (2.5%) - exceptional interest, sometimes considered to be internationally important.
Grade II* (5.5%) - particularly important buildings of more than special interest.
Grade II (92%) - nationally important and of special interest.

There are also locally listed structures (at the discretion of local authorities) using A, B and C designations.

In Scotland three classifications are also used but the criteria are different. There are around 47,500 Listed buildings.
Category A (8%)- generally equivalent to Grade I and II* in England and Wales
Category B (51%)- this appears generally to cover the ground of Grade II, recognising national importance.
Category C (41%)- buildings of local importance, probably with some overlap with English Grade II.

In Northern Ireland the criteria are similar to Scotland, but the classifications are:
Grade A (2.3%)
Grade B+ (4.7%)
Grade B (93%)

Read more at Wikipedia Link