Word Usage – ‘Must’ vs ‘Shall’ Differences Explained

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In Information Technology, the "RFC2119 standard" (not exactly standard, but it does not matter here) provides guidance for the use of some words: "must", "must not", "should", "should not", and "may". The definitions there are very clear and there is no ambiguity.

Then there is the word "shall" that is

used in laws, regulations, or directives to express what is mandatory

Is there any practical difference between "shall" and "must"?

I am not a native speaker of English and in my language (French) you have all kinds of acrobatic constructions to soften or graduate some concepts. This is even more pronounced in some Asian languages.

I wanted to understand if this is the case with "must" and "shall" (= if they mean the same, possibly with some sugar coating) – I do not care in the text I am writing is gentle or not, I just do not want to have any doubts about the coercive meaning (in case there would be, for a reason or another, a "shall").

I understand the formal/dictionary definition above (which clearly states "mandatory"), I am more interested in the "feeling" of these words for an English native speaker.

A practical example (following the comments and answers): "Users must use a password" vs "Users shall use a password". Is there any difference for a native speaker? Any nuance?

Best Answer

In formal and technical language and traditionally in legal language, "shall" is used to express obligation and is no weaker than "must". In fact, in some cases it was formerly preferred to "must" (this may no longer be the case, or may be the case in very restricted scenarios).

In ordinary language, "shall" is sometimes (especially in the first person) simply an expression of the future (or a way of asking for suggestions in first-person interrogatives) - although "shall" is increasingly rare in American English, and even in British English is less used than it once was.

The US government's Plain Language Network advises:

Use “must” not “shall” to impose requirements. “Shall” is ambiguous, and rarely occurs in everyday conversation. The legal community is moving to a strong preference for “must” as the clearest way to express a requirement or obligation.

Surprisingly enough, US courts have ruled that "shall" can have multiple meanings even in a legal context, where it might be thought obvious that it ought to be interpreted in the traditional way.

By contrast, the Alberta Law Review noted in 1994 that "shall" was the preferred word and cited a Canadian legal scholar's view that...

Strictly speaking, ... Everyone must does not directly create a duty; it merely asserts the existence of a duty, however it may have been created. Thus, anyone can say that all motorists must drive on the right, because the legislature has said that they shall.... Hence, in directing commands to persons shall is better.

And:

In contracts and statutes "must" is traditionally used to state conditions of validity that do not directly impose a duty on anyone

But this view is out of date (in most jurisdictions, including Alberta) - to quote the US government's Plain Language Network again:

Australia and at least three Canadian provinces (British Columbia, Alberta and Manitoba) ... have amended their Interpretation Acts to say that “must” is to be interpreted [in legislation] as imperative.

In the UK, there is no suggestion as far as I know that legislative "shall" could be ambiguous, but its meaning is still considered less obvious to the average reader than "must", so the Office of the Parliamentary Counsel, in its guidance on drafting legislation, says:

Office policy is to avoid the use of the legislative “shall”. There may of course be exceptions. One reason for using “shall” might be where the text is being inserted into an Act that already uses it.

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