According to the Concise Oxford Dictionary¹ of Current English²: Sixth Edition, a footnote³ is 'inserted at foot of page'. If you want to know more about what a footnote is, go to the Guide Entry Footnotes. If you want advice on using footnotes, read on...

Do Not Use Irrelevant Footnotes

In theory⁴, a footnote could say anything⁵, as long as it was at the foot⁶ of the page⁷. In practice, footnotes usually contain references⁸, quotes, clarifications and translations⁹. Footnotes can be unrelated to the text, but this is not a good idea. It is a good way to annoy your reader, as they will have looked up the footnote for nothing; they could have waited till they had finished the main text.

Do Not Use Too Many Footnotes

It is easy to use too many footnotes. If you try to use too many footnotes, it can make the text hard to follow. For¹⁰ example¹¹, compare¹² how¹³ difficult¹⁴ it¹⁵ is¹⁶ to¹⁷ read¹⁸ this¹⁹ sentence²⁰, which²¹ is²² full²³ of²⁴ footnote²⁵ references²⁶, to²⁷ the²⁸ previous²⁹ sentence³⁰, which had no footnote references. The first few paragraphs are also an example of using too many footnotes. They have fewer footnote references than the previous example, but they are still quite awkward to read. The problems of having too many footnotes means they should be used as a last resort. If it is possible to work the information into the main text, you should do so. Where footnotes are used, they should be as concise as possible. Long, rambling, footnotes soon make the reader forget what the main text was about.

Do Not Misplace Footnotes

A footnote reference should appear at the first opportunity for that reference. For example, take this quote: 'Mary likes footnotes. Mary collects them in an old sock'. You might want to add a footnote explaining who Mary is. The footnote should appear after 'Mary' in the first sentence, as this is the first time Mary was mentioned.

Conclusions

If you are reading something with footnotes, you will notice that many footnotes are irrelevant, appear too often, and are in the wrong place. This is because many people read instructions purely so that when doing it their own way goes wrong, they can use the excuse 'but I did exactly what it said in the instructions!'. Even people who write instructions often do not follow them. Writers of advice for using footnotes are no exception to this.