The original number of the road between Carnwath and Kirkdean in southern Scotland was the B714. Owing to its importance it was quickly upgraded to A-road status and became the A753 but this was a short-lived number as the whole thing was renumbered A721 in 1935.
Starting on the A70 in Carnwath the B714 headed east across the North and South Medwins (two streams), passing through the village of Newbigging between the two. The road then detoured slightly to the south to avoid the Black Mount. The A702 was crossed at Melbourne at a junction that was then a simple crossroads. On the far side the road crossed a ridge before ending on the A72 to the south of Blyth Bridge.