The simplest possible way of citing is to place the full reference at the relevant place in the text. This makes a lot of sense for short texts with very few citations.
I don't know of any discipline where in-text citations are the norm.
If a reference is repeated, it is rather pointless repeating the full
information every time. Instead, when cited again, authors are
shortened to the surname, titles are shortened to the most distinctive
words, and non-essential information is left out. E.g.,
Khoo Thwe, From the Land of Green Ghosts, HarperCollins,
London, 2002 becomes
Thwe, Green Ghosts or perhaps
Green Ghosts depending on the
A shortened reference works well if the full reference is found nearby, but it obviously can lead to problems if the full reference is found two hundred pages earlier. Therefore, many styles require full references to be given once on each page, spread, chapter or similar.
In order to shortened repeated references even more, there are a few abbreviations in use.
From Latin ibidem "in the same place".. This repeats the previous author and title and whatever else is identical. It is still frequently used.
|Without ibid.||With ibid.|
|Thwe, Green Ghosts, p. 42||Thwe, Green Ghosts, p. 42|
|Thwe, Green Ghosts, p. 42||Ibid.|
|Thwe, Green Ghosts, p. 87||Ibid., p. 87|
|Milton, Paradise Lost, p. 117||Milton, Paradise Lost, p. 117|
|Thwe, Green Ghosts, p. 87||Thwe, Green Ghosts, p. 87|
|Thwe, Green Ghosts, p. 42||Ibid., p. 42|
From Latin idem "the same". This repeats the previous author. It is rarely used today.
From Latin opere citato "in the work cited". This repeats the previous title for a given author. It is not often used today.
From Latin loco citato "in the place cited". This repeats the title and page number for a given author. It is not often used today.
|Without loc.cit.||With loc.cit.|
|Pascal Khoo Thwe, From the Land of Green Ghosts, p. 42||Pascal Khoo Thwe, From the Land of Green Ghosts, p. 42|
|John Milton, Paradise Lost, p. 117||John Milton, Paradise Lost, p. 117|
|Thwe, Green Ghosts, p. 87||Thwe, op.cit., p. 87|
|Thwe, Green Ghosts, p. 87||Ibid.|
|Milton, Paradise Lost, p. 117||Milton, loc.cit.|
|Thwe, Green Ghosts, p. 87||Thwe, loc.cit.|
|Thwe, Green Ghosts, p. 42||Ibid., p. 42|
Even with shortened references and ibid.-style abbreviations, having many references will make the text difficult to read. Therefore, references are often moved into footnotes or endnotes. (This is independent of the use of shortened references and/or ibid.-style abbreviations.)
Citing in footnotes is popular in law, while endnote citations seem to be popular within history.
One might argue that citing in endnotes is reasonably close to having an annotated numbered bibliography in citation order, except that the latter will not contain repeated references.
However, most numbered bibliographies are in alphabetical order.
In the text, just a number is given, normally in square or round brackets or as a superscript number (the latter seems to be preferred in physics), i.e., "... as shown in the TeXbook ", "... as shown in the TeXbook (17)" or "... as shown in the TeXbook17".
Although this way of citing is very clear,
it has the problem that the reader has no way of knowing
what "" means without looking it up in the bibliography --
looking down to the footnote or guessing what (Knuth 1987) might mean
would generally be quicker.
This is no problem if the author drops hints in the main text
as to what s/he is quoting, but personally I always get irritated
when I see something like
It is shown in  that ....
Numerical citations are used a lot within mathematics, physics and computer science.
Technically, alpha citations work just as numerical citations. Instead of a number, however, a unique label is generated for each title, normally based on the three first letters of the surname if there is only one author, or the first letter of each surname if there are more authors, followed by the two last digits of the year (note, however, that slightly different ways of constructing the label do exist). For instance, Knuth 1987 -> [Knu87], Thwe 2000 -> [Thw00], Devoto & Giacomelli 1971 -> [DG71].
Alpha citations seem to be popular in Germany.
Author-year citations dominate within linguistics.
Listing the publication year along with the author really only helps if s/he is cited with more than one title, and one has to escape to additional letters if a productive author has published more than one work per year.
The author-number system escapes this problem by
This style was mentioned to me by Frank Mittelbach. I have yet to see it used.