Plain is the name for what has no affinity. It is not another affinity, and it has no place on the ring where the five affinities feed and repel each other. It covers ordinary folk who settle the ground and pay geld, drilled soldiers, and every made thing.
Some spells are Plain too, open to every casting mage whatever their affinity. A Plain spell still needs a casting core, so coreless Human lords cannot cast it.
Five of the ten ancestral arts are Plain: Lasting Shape, Banner, Weaver's Call, Reaping and Warding. Plain training has no grain, so no core ever trains one of them against its own affinity. These five are also the home of the will, the trained resolve of the coreless, which holds them at ordinary effort.