The B6438 is a cross-country road in east Berwickshire. Originally the road was unclassified west of the A1 and numbered B1344 to the east. The unclassified section was given the B6438 number around 1930, then the road was extended to St Abbs in 1935.
The road starts on the A6112 a mile or so north of Preston and heads northeast, contouring along a valley side. We pass a turn to the isolated Bunkyl church and then continue through a few hamlets before bypassing the village of Auchencrow. The B6437 then comes in from the right and is the dominant number in a multiplex ahead, which ends at the top of a hill where our road turns right.
We now descend into the valley of the Eye Water. It's not long before we reach Reston, one of the largest places en route, where we run along Main Street. A bridge over the East Coast Main Line separates the two halves of the village. After running through the village centre we bear left to cross the Eye Water and reenter open country. We meet the A1 shortly afterwards and cross it at a staggered crossroads; that road is only S2 at this point.
Now heading north we descend and climb through a series of valleys and presently reach Coldingham and a T-junction on the A1107. There's then a multiplex to the left (although we keep the alignment of our road), which ends when the A1107 bears sharply left and the B6438 continues ahead. We run along the shared space of the High Street before descending towards the sea. A sharp left-hand bend takes us across a burn after which there's a right turn to Coldingham Sands. We continue ahead, however, with the coast clearly visible to the right. It's not long before the road bends sharply right and we reach St Abbs. The road ends at a road junction by the visitor centre; a narrow road continues ahead to the harbour.