Polyunsaturated fats are triglycerides in which the hydrocarbon tails constitutes polyunsaturated fatty acids (PUFA) (fatty acids possessing more than a single carbon–carbon double bond). Polyunsaturated fat can be found mostly in nuts, seeds, fish, algae, leafy greens, and krill. "Unsaturated" refers to the fact that the molecules contain less than the maximum amount of hydrogen. These materials exist as cis or trans isomers depending on the geometry of the double bond.
Saturated fats have hydrocarbon chains which can be most readily aligned. The hydrocarbon chains in trans fats align more readily than those in cis fats, but less well than those in saturated fats. This means that, in general, the melting points of fats increase from cis to trans unsaturated and then to saturated.