The B721 runs along the original line of the A75 between Annan and Gretna.
It starts on the A75 at the western end of the Annan bypass. It then heads east into town, meeting the B723 and B724 in quick succession before crossing the River Annan into town. It runs along the High Street, meeting the B722 at traffic lights before reaching a roundabout and the B6357 on the far side of town.
Now in open country (although a surprising number of buildings still line the road) the road continues east, crossing the Carlisle to Dumfries railway line and entering Dornock. There is then a short rural section before Eastriggs, built to house workers at the nearby ammunitions factory during World War I.
The road continues east across the flat coastal plain to Rigg, where it crosses the Kirtle Water before running alongside the A75 for the best part of a mile until a short link road turns off to the north to connect the two roads. The B721 then heads into Gretna. It runs through town along Annan Road to end at a roundabout on the B7076. At one time the road continued to the right to cross the River Sark into England and end on the A74. When that road was upgraded online to motorway status the B7076 local access road took over this easternmost section of the B721.
The original route of the B721
As stated above, the current route of the B721 is entirely the pre-bypass A75
. The original B721 did exist in the Gretna area, however.
As shown in the map to the left, it started on the A75 to the west of Gretna and headed around the north of town. Following a short multiplex along the A74 it continued into Gretna Green where it ended on the original line of the A6071.
The northern section of the original B721 is now part of the B7076 whilst the rest is unclassified.