Fatty acids consist of a long chain of carbon atoms that are joined to hydrogen atoms. If the carbon chain is linked to the maximum number of H atoms with no double bonds it is saturated, because no more H atoms can be added. The length of a fatty acid can vary

If the chain contains a double bond between two of the carbon atoms it is unsaturated.

A chain with just one double bond is monounsaturated, while one with two or more double bonds is polyunsaturated.

Polyunsaturated fatty acids tend to be liquids at room temperature and are mainly from plant sources, e.g. sunflower oil and olive oil.

Unsaturated fatty acids may be either cis or trans configuration. If the spaces where additional hydrogen atoms could bond are both on the same side of the fatty acid it is known as a cis fatty acid and the carbon chain is slightly bent. If the spaces are on opposite sides, it is a trans fatty acid, which has a straight chain.