What typographic rules are there for indenting paragraphs?

Question

I wonder what typographic rules there are for indenting. Does it require a certain amount of lines to be justified or should it be applied per default.

In my opinion, the text colour is not very nice, when having two-liners as in my example.

But what are the more qualified opinions on this?

Answer 1

I'm not qualified, but I can give an opinion.

You definitely should not change the paragraph style based on its length, as that will produce a very uneven appearance and confuse the reader. However in documents with many one or two line paragraphs (often the case with technical manuals) it often works better to give up on indentation and justification and set the text ragged right with no indentation and perhaps some vertical \parskip instead.

Answer 2

Unless you're German (like me), indentation is a total must, even for two-liners - according to German typography authority Jan Tschichold:

"The beginnings of paragraphs must be indented. Paragraphs without indent (unfortunately the rule in Germany, and only there) are a bad habit and should be eliminated. The indention – usually one em – is the only sure way to indicate a paragraph. The eye, on reaching the end of a line, is too inert to recognize a tight exit – and in works without indents, even that frequently has to be produced as an afterthought from a flush ‘last’ line. In order of importance, legibility and clarity have to come first; a smooth contour of the typeset page is of lesser importance. Therefore, typesetting without indentions is to be dismissed as an error."

Source: Tschichold, Jan (1997): The Form of the Book: Essays on the Morality of Good Design, p.17.