When is textwidth 4.5in in latex

bookslatex-basepagetextwidth

In latex documentation, I found this:

First Image

Then I searched that what does bk mean, and I found this:

Second Image

After seeing this I thought that bk means book. And textwidth equals to 4.5in in book class (font size 10pt). Then I compiled a pdf with papeeria:

Third Image

And this equals to 345pt, but the docs said it would be 345 if it is not book class. But it is still 345pt. Then I thought it might be possible that 4.5in equals to 345pt. But I found this:

Forth Image

Can someone tell me what I am understanding wrong?

Best Answer

bk is a doctsrip guard. If you look at the file classes.ins in the latex base distribution then you will see

          \file{bk10.clo}{\from{classes.dtx}{10pt,bk}}

showing that the book 10pt option file bk10.clo is generated using the bk guard.

The code you show is not for documents starting \documentclass{book} but for documents starting \documentstyle{book} which triggers compatibility mode for documents written for LaTeX 2.09 and so ensures exactly the same sizes as you would have got using LaTeX 2.09 in 1992 before LaTeX2e was released.

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