English: Carbon numbering schemas for fatty acids:

(1) systematic IUPAC numbering ("C-1", "C-2", "C-3", ...) starting from 1 at the carboxyl carbon (blue).

(2) old Greek letter labels ("α", "β", "γ", "δ", ...) starting at the carbon AFTER the carboxyl (green)

(3) omega-minus numbering ("ω", "ω−1", "ω−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" acid with 18 carbons (as shown) has the bond between carbons 18−3 = 15 and 18−2 = 16.

A common mistake is to say that the last carbon is "ω−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" is at carbon 15, not 16. See for example this article.

Note also that the "−" in the omega-notation is a minus sign, and "ω−3" should in principle be read "omega minus three". However, it is very common (especially in non-scientific literature) to write it "ω-3" (with a hyphen/dash) and read it as "omega-three".