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The present bridge dates from 1624 and is the fourth to have stood on this location. Two of the previous structures were destroyed by flooding and one by an English attack. The bridge is 355 metres long and was the original route of the A1, before the construction of the Royal Tweed Bridge in the 1920s. The bridge is a Grade I listed structure. Link

The River Tweed is 97 miles long and flows generally west to east through the Scottish borders. It rises at Tweedsmuir and passes through small towns like Peebles, Selkirk, Melrose, Galashields and Kelso, before entering the North Sea at Berwick on Tweed.

Built in the 1920s to divert traffic off the older Berwick Bridge across the River Tweed. It is a reinforced concrete bridge. Until the bypass was built in the 1980s it carried the A1. Grade II* listed. Link

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

Grade I listed buildings and structures are of exceptional, even international importance. There are over 6,000 in the country. Only 2.5% of listed buildings are Grade I listed.
In Scotland the classification is Category A
Index: Link