|Length:||2.1 miles (3.4 km)|
|Old route now:||A891|
|Route outline (key)|
As described below, the B821 in south Stirlingshire is a mere shadow of its former self, indeed what survives wasn't even originally classified in 1922..
Today's route starts on the A81 at Blanefield, the north western end of Strathblane, and heads west along Station Road. The railway is long gone, although its former route is still visible on the far side of the Blane Water. After leaving the village, the road climbs through fields, and then turns sharply left at Ardoch. Shortly after, it turns back to the right; this climbs the steepest part of the hill, with the gradient easing once more. Running through woodland now, the road is crossed by the West Highland Way at Carbeth Loch. The final few hundred metres are downhill, to the junction with the A809 just to the north of the Carbeth Inn.
In 1922 the road ran east from Strathblane via Milton of Campsie to the A803, loosely following the Strathkelvin Railway Branch Line. However, sometime before 1932 this road became the A891 throughout. It is not currently known whether the B821 had already been extended to Carbeth (via a short multiplex with the A81) or whether the number was quickly reallocated. Either way, the B821 is now just the short route we have today.