[texhax] \write17?

[Hefferon, Jim S.]

I'm trying to understand the \write<number> command, specifically \write16 and \write17.  I'd appreciate any help.

I understand that the output files are ordinarily numbered  1 to 15, and that if <number> is positive and outside that range then TeX writes to both the log file and the terminal.  So I think I understand that \write16 was adopted  early on as a convention to write to both.  I see it in the TeXbook.  Is that understanding right?  

But \write17?  Google gives me a references to it, but what is it for?   How is it different than \write16?   If it is no different then why was it skipped over in getting to \write18?

(I know that \write18 is an entirely different animal.)

Thank you,
Jim

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