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?

1 Answer 1


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


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.

  • Omg, it worked. However, is documentstyle old but still usable because lots of documents already have been written with this or is it new feature added to latex? Feb 6, 2022 at 13:01
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    @rajin100000 \documentstyle has always been in latex latex2.09 did not have \documentclass or \usepackage so if you start a document with \documentstyle latex disables the use of \documentclass and \usepackage and tries to make things work like it did in the1980s, so at the time latex2e came out in 1993 all existing documents started with \documentstyle now essentially none do, unless you dig out an old archive tape from last century. Feb 6, 2022 at 13:03
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    never try and write a new document using \documentstyle. As David says it tries to get you back into 80ties and and 80-90 percent of what LaTeX does and can do today is gone (and none of the more modern packages are likely to work at all). It is really just for being able to recompile (with fingers crossed) an unchanged document from that time. Feb 6, 2022 at 14:41

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