The chemical composition of pig backfat determines its physical properties of firmness and cohesiveness as well as its nutritional value. The firmness of the tissue depends upon its content of saturated fatty acids, particularly stearic, which make it harder; and unsaturated fatty acids, particularly linoleic, which make it softer. Cohesiveness depends upon the connective tissue and fat content and the compositon of these; soft fat and low cohesiveness frequently being found together. Nutritional value may be judged by the energy content of the tissue, which increases in proportion to its lipid content, or by the ratio of polyunsaturated fatty acids to saturated fatty acids (P:S ratio), used by COMA to indicate the desirability of the fat as a risk factor for cardiovascular disease. There is evidence that as fatness has been reduced in British pigs, composition has changed with consequences for these characteristics.