The C21 is a short link in Selkirk, meeting the A7 at both ends. Curiously, the route described in the council list is only about half the length stated, which is 1.3 miles. It is not clear if this is an error, or if the route includes loops or spurs which are not fully defined in the description. As described, the route starts on Tower Street in the town centre and heads north east along the High Street, and on to Bleachfield Road. It then winds steeply down hill, taking Raeburn Lane and Shawburn Road into Raeburn Place to rejoin the A7.