The common south of the Heads of the Valleys road towards Bargoed is owned by the Marquis of Bute and that north of the road towards the Brecon Beacons by the Duke of Beaufort.

In exchange for grazing rights each commoner is responsible for the upkeep of the land, its drainage, fertilisation, spraying and ensuring that the area is not overgrazed. Animals that have been illegally grazed can be impounded and their owners made to pay a fee before they are returned. Commoners are allowed to graze seven sheep or one cow or horse for each acre held.

Local breeds of sheep are predominantly Glamorgans or Cheviots which are crossed with Welsh mountain sheep. Glamorgan crosses are grazed on shale based hills, such as those that have coal beneath, and Cheviot crosses on limestone based hills which are to be found north of the Heads of the Valleys road around Trevil and the white stone areas of Blaen Rhymney.

Earmarks which belong specifically to the holding and from which they cannot be separated identify sheep. The left, right or both ears may be marked with a single or combination of cuts, notches, splits, punchholes, hacks and spittle marks which identify the animal's owner.

The seasonal turning out of the tups (male sheep) with the ewes is crucially timed in order that lambing does not take place too early in the year, late autumn being favoured in the valley farms. The old adage that "when the tups are turned out at Guy Fawkes, the ewes will have an April Fool's lamb" gave a useful yardstick from which gestation could be judged. It is important that upland lambs, born in early spring, should have a good summer of grazing ahead of them during their first year and that ewes are wormed and sheared to maintain their health and milk producing capacity.