A relatively highly unsaturated oil which, on exposure to air, is oxidized and polymerized to form a hard, dry film. Vegetable and fish oils are classified as nondrying, semidrying, or drying, according to their ease of autoxidation and polymerization. Due to their film-forming properties, drying oils are used in paints, inks, and varnishes. The reactivity of drying oils with oxygen results from the presence of diallylic groups (that is, two double bonds separated by methylene groups, –CH=CHCH₂CH=CH–) or conjugated double bonds (two carbon-carbon double bonds separated by a single bond). The number of diallylic groups fn per molecule is correlated with the oils' drying nature. Oils with fn greater than 2.2 are drying oils; those below 2.2 are semidrying to nondrying. The table shows the percentage of saturated and unsaturated fatty acids and the fn for some drying oils. These oils vary considerably by source and harvest time (see “Sunflower” in the table).