2.1.4 Positions of double bonds are specified by superscript numbers following a 4 symbol,eg, linoleic acid(亚油酸)is abbreviated as 18: 2(49 12). 2. 1. 5 The number of carbons in a fatty acid is commonly even and the positions of carbon- carbon double bonds are regular (usually at positions 9, 12, 15(+3), separated by a methylene roup and never conjugate, that is, alternating gI single and double bonds).(cis).2.1.4 Positions of double bonds are specified by superscript numbers following a  symbol, e.g., linoleic acid（亚油酸）is abbreviated as 18:2(9,12). 2.1.5 The number of carbons in a fatty acid is commonly even and the positions of carbon￾carbon double bonds are regular (usually at positions 9, 12, 15 (+3), separated by a methylene group and never conjugate, that is, alternating single and double bonds). (cis)