1. Systematic IUPAC numbering (${\displaystyle C-1,C-2,C-3,...}$) starting from 1 at the carboxyl carbon (blue).
2. Old Greek letter labels (${\displaystyle \alpha ,\beta ,\gamma ,\delta ,...}$) starting at the carbon after the carboxyl (green).
3. Omega-minus numbering (${\displaystyle \omega ,\omega -1,\omega -2,...}$) starting at the end opposite to the carboxyl (red).

The position of a double bond is always the carbon closest to the carboxyl, even in the omega numbering. Thus a ${\displaystyle \omega -3}$ acid with 18 carbons (as shown) has the bond between carbons ${\displaystyle 18-3=15}$ and ${\displaystyle 18-2=16}$.

A common mistake is to say that the last carbon is ${\displaystyle \omega -1}$. Another common mistake is to say that the position of a bond in omega-notation is the number of the carbon closest to the end. For double bonds, these two mistakes happen to compensate each other. However, for substitutions and other purposes, they don't: a hydroxyl at ${\displaystyle \omega -3}$ is at carbon 15, not 16.^[1]

${\displaystyle \omega -3}$