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If the two bromines are attached at opposite ends of the molecule, why are they both labelled with a 6?

Also, why is 6,6′-dibromoindigo not called 1,1′-dibromoindigo if carbons in a ring are numbered according to their substituents, and bromine is the substituent, therefore the carbon it is attached to should be called 1, not 6?

Structure of Tyrian purple
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    $\begingroup$ Because of Indole numbering: N is 1 and the other atoms (except the C that bridges both rings) are numbered in ascending order, so the Br substituent is at position 6 in both rings. $\endgroup$
    – Aniruddha Deb
    Oct 26, 2020 at 4:21
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    $\begingroup$ Br atoms are not both numbered 6. The other one is 6', where the apostrophe means the same position but on the other indol structure. $\endgroup$
    – Poutnik
    Oct 26, 2020 at 6:43
  • $\begingroup$ Beside the above comments, rings numbering is always, although "fixed" and coded, quite a "mess". $\endgroup$
    – Alchimista
    Oct 26, 2020 at 9:50

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