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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

The Leeds and Liverpool Canal is the longest canal in Northern England at 127 miles long. The first of the trans-Pennine canals it took 46 years to build at a cost of five times the original budget, mainly because of the length and complexity of the route. It passes through 91 locks with a summit level of 487 feet at Foulridge near Nelson and Colne. It was originally conceived in the 18th century to carry woollen goods from Leeds and Bradford and limestone from Skipton but in its 19th century heyday it carried stone, coal and many other goods. The impact of the railways was not as great as with other canals and commercial traffic continued along the main canal until 1964. Regular work stopped in 1972 when the movement of coal to Wigan Power Station ceased. In the latter part of the 20th century the leisure potential of the canal was developed and it is now a popular destination for cruising, fishing, walking and cycling. See Link for detailed information.