The road starts at traffic lights on the northern edge of Abergavenny town centre, where the A40 turns left. We head straight on along Hereford Road, the pre-bypass route of the A465. Shortly Park Crescent comes in from the left; it appears that this was the original line of the B4521. Our road then TOTSOs right along Grosvenor Road then winds to the left and right before going under the railway line and adjacent A465. There's a left turn shortly afterwards, marked on maps as a spur of the B4521, which allows access to/from the A465.
We continue eastwards and pass a hospital and golf course before entering open country proper. We head towards the wooded pointy hill of Skirrid Fawr before bearing right to find out the hill is actually more of a ridge. We pass Llanddewi Skirrid and continue to zigzag to the east, passing through a few villages.
At Norton the B4347 crosses us at a staggered crossroads, then we descend into Skenfrith. After passing to the south of the castle, now owned by the National Trust, we cross the River Monnow. That river runs along the border between England and Wales for much of its route but at this point the border runs straight to avoid a meander and so we remain in Wales until we bear left to pass through a wood. After climbing out of the valley we run through Broad Oak and then continue on to cross the A466.
Now less winding, the road continues in a vaguely northeasterly direction and soon reaches St Owen's Cross, where it crosses the A4137. We continue on for another few hundred yards to the end of the road at a T-junction on the A49 a few miles west of Ross.