What Do the Indices 6 and 13 Stand for in 6,13-Pentacenequinone?

[Pete99]

What do the indices 6 and 13 stand for in 6,13-pentacenequinone?

I know that is related to the position of the substituents in the molecule, but I cannot figure out how to interpret it.

Can some one point me to a reference where the basics the nomenclature are explained?

Thanks,

 

[aroc91]

This seems to cover it pretty well.

http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/nomen1.htm

We didn't cover things like this in O chem, but I can make a slightly educated guess, in hopes that someone more knowledgeable will come along.
Edit: I did a bit more research and got it. The carbons are numbered atypically. Oh well, that's typical IUPAC for you, I suppose.

The picture is now correct, showing that carbon 6 and carbon 13 are the ones with the oxygens attached.

 

[Pete99]

Thanks! Now it is clear.