Brecknockshire (Sir Frycheiniog) includes the Brecon Beacons and part of the Black Mountains which it shares with the county of Monmouthshire and is part of the administrative area of Powys.

The Black Mountains, part of the Brecon Beacons National Park, reach about 2660 feet at the highest point with narrow mountain roads winding through wild uplands. These mountains are made of Old Red sandstone (Devonian) which are about 400 million years old. The Brecon Beacons are just over 2900 feet at the highest point of Pen-y-Fan.

This county borders England to the East.