The right and wrong side of paper. WC brushes and oil paint. This means it is made from pure cotton fibers. This guarantees that the paper will not deteriorate or yellow with age. The other important difference is the sizing.
Paper contains an ingredient called sizing.
This controls the amount and rate it will absorb water. High quality papers have a large amount of sizing that is guaranteed to be the same from sheet to sheet. That gives you, the painter, a way to control the fluid paint on the surface. Burnt Sienna, Yellow Ochre, Burnt Umber, Cobalt Blue, Ultramarine Blue, Alizarin Crimson, Cerulean Blue, Naples Yellow, Cadmium Red, Cadmium Yellow, Paynes Gray, Viridian, and Hooker Green. Get the more expensive bigger ones when you see you are really going to stick with it.