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Our advice on asking questions suggests that you prepare a “minimum example” (also commonly known as a “minimal example”) of failing behaviour, as a sample to post with your question. If you have a problem in a two hundred page document, it may be unclear how to proceed from this problem to a succinct demonstration of your problem.

There are two valid approaches to this task: building up, and hacking down.

Building up starts with a basic document structure (for LaTeX, this would have \documentclass, \begin{document}, \end{document}) and adds things. First to add is a paragraph or so around the actual point where the problem occurs. (It may prove difficult to find the actual line that’s provoking the problem. If the original problem is an error, reviewing “the structure of TeX errors” may help.)

Note that there are things that can go wrong in one part of the document as a result of something in another part: the commonest is problems in the table of contents (from something in a section title, or whatever), or the list of <something> (from something in a \caption). In such a case, include the section title or caption (the caption probably needs the figure or table environment around it, but it doesn’t need the figure or table itself).

If this file you’ve built up shows the problem already, then you’re done. Otherwise, try adding packages; the optimum is a file with only one package in it, but you may find that the guilty package won’t even load properly unless another package has been loaded. (Another common case is that package A only fails when package B has been loaded.)

Hacking down starts with your entire document, and removes bits until the file no longer fails (and then of course restores the last thing removed). Don’t forget to hack out any unnecessary packages, but mostly, the difficulty is choosing what to hack out of the body of the document; this is the mirror of the problem above, in the “building up” route.

If you’ve added a package (or more than one), add \listfiles to the preamble too: that way, LaTeX will produce a list of the packages you’ve used and their version numbers. This information may be useful evidence for people trying to help you.

The process of ‘building up’, and to some extent that of ‘hacking down’, can be helped by stuff available on CTAN:

• the minimal class (part of the LaTeX distribution) does what its name says: it provides nothing more than what is needed to get LaTeX code going, and
• the mwe bundle provides a number of images in formats that (La)TeX documents can use, and a small package mwe which loads other useful packages (such as blindtext and lipsum, both capable of producing dummy text in a document).

What if none of of these cut-down derivatives of your document will show your error? Whatever you do, don’t post the whole of the document: if you can, it may be useful to make a copy available on the web somewhere: people will probably understand if it’s impossible … or inadvisable, in the case of something confidential.

If the whole document is indeed necessary, it could be that your error is an overflow of some sort; the best you can do is to post the code “around” the error, and (of course) the full text of the error.

It may seem that all this work is rather excessive for preparing a simple post. There are two responses to that, both based on the relative inefficiency of asking a question on the internet.

First, preparing a minimum document very often leads you to the answer, without all the fuss of posting and looking for responses.

Second, your prime aim is to get an answer as quickly as possible; a well-prepared example stands a good chance of attracting an answer “in a single pass”: if the person replying to your post finds she needs more information, you have to find that request, post again, and wait for your benefactor to produce a second response.

All things considered, a good example file can save you a day, for perhaps half an hour’s effort invested.

Much of the above advice, differently phrased, may also be read on the web at http://www.minimalbeispiel.de/mini-en.html; source of that article may be found at http://www.minimalbeispiel.de/, in both German and English.