Chinese Wall
'Chinese Walls' are the supposed communications barriers between members or departments of the same financial institution.
They are used to ensure that price sensitive information is not leaked - say from a corporate finance department involved in a takeover, to a dealer who would be in a position to buy shares in the companies involved.
The Great Wall of China failed to keep the barbarians out - despite being massive and (apparently) the only human construction visible to the naked eye from outer space. It exists and it failed.
Chinese Walls in the city do not exist, they are imaginary but taken seriously. In some City investment houses, they appear to work, in others there are doubts whenever there are 'unusual share price movements'. These movements are not widespread but more common than many would like to believe. When they do occur people suspect ' insider dealing'
Decide for yourself how effective you think these Chinese Walls are!