RAG - the two main rag fibres used in paper making are cotton and linen, rag paper consists of 25-60% rag fibre and the rest is chemical wood pulp.

Ream - a term denoting a number of sheets of paper ranging from 480 to 516, most commonly 500.

Reel - Term used for continuous roll or web of printing paper.

Relative Humidity - the mass of water vapour actually contained in a given volume of air expressed as a percentage of the maximum mass of water vapour that could be contained in that same volume at the same temperature.

Retention - the percentage of the amount of a loading or additive added to the stock that is retained in the final sheet of paper.

Rigidity - the force required to bend a strip of paper or board through a known angle.