Sometimes there's no other way than to specify locants of the both carbons of the double bondWhen using general IUPAC rules for locant number minimization, whenone might be tempted to use the singlecompound locant specification would be ambiguous. Especially when there's predefined fixed numberingfor some rather simple cases like multi-substituted cyclohexenes, like in natural compound skeletons:
pinane2,6,6-trimethylbicyclo\[3.1.1\]heptane
pin-2(10)-ene(*older name layout:* 2(10)-pinene)(*traditional non-systematic name:* β-pinene)
6,6-dimethyl-2-methylidenebicyclo\[3.1.1\]heptane‘cyclohex-1(See usage in chemical papers: Cyclobutane ring opening of pin-2(10)-ene with mercury(II) salts. A new, high-yield synthesis of p-mentha-1,8-dien-7-ol or Some free-radical addition reactions of pin-2-ene, pin-2(10)-ene, and thuj-4(10)-ene.6)
Another example:-ene-1,2,4-triol’ instead of
cyclohex-6-ene-1,2,4-triol
Δ6a(10a)-tetrahydrocannabinol(*sometimes with incorrect punctuation:* Δ6a,10a-tetrahydrocannabinol)6,6,9-trimethyl-3-propyl-7,8,9,10-tetrahydro-6H-dibenzo[b,d]pyran-1-ol(more on this notation also in Q: This Use of Δ (delta) to describe alkenes(x)
Thisy type of locant is called compound locant in IUPAC Nomenclature of Organic Chemistry (Preferred names (2013)) terminology:
When using general IUPAC rules for locant number minimization, one might be tempted to use the compound locant for some rather simple cases like multi-substituted cyclohexenes, like the 2nd wrong(?) name in:
cyclohex-6-ene-1,2,4-triol(*not* cyclohex-1(6)-ene-1,2,4-triol)(it's not the IUPAC citation, but result of almost all IUPAC naming software)
Why ‘cyclohex-1(6)-ene-1,2,4-triol’ is most likely wrong might be concluded from the following rule
So, IUPAC organic nomenclature rules tend to avoid these compound locants (x)y where it's possible. But, as Ivan Neretin has stated in his comment, for monocyclic compounds, there even is no ambiguity at all.