There are a few variations, especially with electronic information, but they all follow the same basic structure.
If the author has written more than one paper in the same year, then you can use an alphabetical appendix: The other difficulty is when there is no author mentioned, and the source was written by an organization.
If you can, try to find the original source and use that.
If you cannot obtain this source, then you need to use a two-stage referencing system.
For example If there are six or more authors, you can use the abbreviation straightaway.
Occasionally, you may have to use a source that has been referenced in another source.
If you use class notes, some lecturers are not too worried about citations, although it is usually good practice to find a source saying the same information, from a textbook or journal.
The text in this article is licensed under the Creative Commons-License Attribution 4.0 International (CC BY 4.0).
The exact abbreviation does not matter too much, as long as it is clear in the bibliography.
The same is true of many electronic sources, although be careful that any non-attributed source is reliable.