File:Fatty acid carbon numbering.svg

Systematic IUPAC numbering (

       C
       −
       1
       ,
       C
       −
       2
       ,
       C
       −
       3
       ,
       .
       .
       .
     
   
   {\displaystyle C-1,C-2,C-3,...}
 

) starting from 1 at the carboxyl carbon (blue). Old Greek letter labels (

       α
       ,
       β
       ,
       γ
       ,
       δ
       ,
       .
       .
       .
     
   
   {\displaystyle \alpha ,\beta ,\gamma ,\delta ,...}
 

) starting at the carbon after the carboxyl (green). Omega-minus numbering (

       ω
       ,
       ω
       −
       1
       ,
       ω
       −
       2
       ,
       .
       .
       .
     
   
   {\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

       ω
       −
       3
     
   
   {\displaystyle \omega -3}
 
acid with 18 carbons (as shown) has the bond between carbons 
 
   
     
       18
       −
       3
       =
       15
     
   
   {\displaystyle 18-3=15}
 
and 
 
   
     
       18
       −
       2
       =
       16
     
   
   {\displaystyle 18-2=16}
 

. A common mistake is to say that the last carbon is

       ω
       −
       1
     
   
   {\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

       ω
       −
       3
     
   
   {\displaystyle \omega -3}
 
is at carbon 15, not 16.[1]

Note also that the "−" in the omega-notation is a minus sign, and

       ω
       −
       3
     
   
   {\displaystyle \omega -3}