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In the user's guide for the amsmath package (Version 2.1) on page 3, the descriptions for the options leqno and fleqn are as follows:

leqno: Place equation numbers on the left.

fleqn: Position equations at a fixed indent from the left margin rather than centered in the text column.

However, the meaning of the abbreviations "leqno" and "fleqn" are not immediately clear from these descriptions. In a response on this link, the following meanings were suggested:

leqno: left equation numbers

fleqn: flush left equations

According to Merriam-Webster Dictionary's entry for "no," which states that it is derived from the Latin word "numero" (ablative of "numerus"), it can be understood as meaning "number."

Therefore, it can be inferred that leqno stands for "left equation no". However, it is unclear what the "n" in fleqn stands for if it is interpreted as "flush left equations."

I was not able to find a definitive explanation for the abbreviations leqno and fleqn and their corresponding words through a search engine. It seems that the suggested meanings are based on reasonable deductions/guesses.

PS: English is not my native language, so please forgive any typos or inaccuracies in my expression. If there is any unclear part of my question, please let me know, and I will be happy to rephrase it until we can communicate effectively. Similarly, I am willing to do what I can to help in the process of finding an answer.

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  • I am going to guess that fleqn does stand for flush left equations i.e. "flush left equations". In English, the letters for an abbreviation do not need to immediately follow each other so I think this is the same too. eqn is typically used as the abbreviation for equation in my previous experience but I do not know the internals of amsmath so cannot be authoritive
    – JamesT
    Apr 19 at 11:43
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    @JamesT Thank you, even if your answer may not be authoritative, your experience still resolved most of my confusion on this question. Perhaps only the author of amsmath can give an authoritative explanation. Personally, I even feel that this question has already been resolved.
    – Yangen Pan
    Apr 19 at 12:08
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    Note that fleqn exists also in the kernel and amsmath just redefines it, so the question is probably unrelated to amsmath.
    – campa
    Apr 19 at 12:33
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    Actually, the "fl" in \fleqn does not mean "flush to the left margin", at least in amsmath. The "fixed indent" is meaningful, and it has the width \mathindent (default 30pt).
    – barbara beeton
    Apr 19 at 15:03

1 Answer 1

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As shown in C:\texlive\2023\texmf-dist\tex\latex\base\leqno.clo:

left equation numbers

As shown in C:\texlive\2023\texmf-dist\tex\latex\base\fleqn.clo:

flush left equations

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    \fleqn in amsmath means "flush left to a fixed indent", \mathindent (default = 30pt).
    – barbara beeton
    Apr 19 at 15:08
  • @Clara Thank you for your answer! I have marked your answer as the correct one, but as my reputation is less than 15, I am unable to upvote your answer.
    – Yangen Pan
    Apr 19 at 15:11

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