"must" vs "shall" - are they the same, or is one a softened version of the other?

Question

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?

Answer 1

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.

Answer 2

The first thing to note is that language used in specific fields such as Information Technology is jargon, and does not necessarily carry the same meaning as dictionary definitions of the words. Your document is defining these words specifically as they apply to the RFC process which is part of the ITIL methodology. I will address the English meaning of the words first, then return to your stated context.

In English grammar, "must" denotes compulsion. If you must do something, it means you are expected to, required to, or compelled to.

"Shall" is just a more formal way of saying "will", only used when the subject is "I" or "we" (ie "you will", "we shall"). It does not carry the same inference of compulsion.

For example, "I will go to the store tomorrow" is just a statement of intention; whereas "I must go to the store tomorrow" infers that you have a purpose or a reason why going tomorrow is important.

Having said that, orders made with "will" or "shall" (for example, "you will clean your room!") could be said to mean that you must carry out the order.

The meaning of these words as set out in your document is not meant to override the dictionary definitions of the words, and the scope of these definitions is limited to RFCs in ITIL methodology. The purpose of defining the words is so that they can be used to clearly indicate requirement levels associated with each element of the request for change. There are many other words used in ITIL which have specific meanings in the process and do not match their English language counterparts. The example I always cite is "unresolved", which in incident management means to reverse the process of marking a ticket as resolved. Of course, in English, "unresolved" means that something has never been resolved. In short, don't be surprised if jargon in a field of business does not make grammatical sense.

Answer 3

Here is the basic difference, native speaker-wise:

As found in orders or instructions, in formal language, including technical language. must is to have to do, but more formal.

• All students must enter the gym through the side door this week.
• Lever B must be upright before the door is closed.
• Applications must be filed before June 2nd.

Shall is no longer used in sentences like the ones above to signal an order or instructions.

Must here in French is devoir or Il faut que etc.

As found in everyday language where one person tells another they have to do something. To have to do something = semantically, must do something.

• You must go back to the office now or your boss will find out your are out.

• You have to go back to the office now or your boss will find out you are out.

• We must finish this job by 5 o'clock.

• We have to finish this job by 5 o'clock.

• He must do it immediately, mustn't he?

• He has to do it immediately, doesn't he? [has got to do it, hasn't he]

• Must you keep doing that?

• Do you have to keep doing that?

In this sense, must and shall are not related.

past tense: He must have seen the dog run out the door. [devoir avoir + participe passé]

In legal language, shall is still used to express a binding obligation in a contract or treaty or those kinds of documents and contexts, and must would never be used there in a contract or treaty. Must is not much used in contracts, though, of course, it might be. That said, shall in contract means must in everyday language:

• The parties shall meet once a week for the first six months of operation.

Here is an example of a treaty between the US and Turkey where it is used 58 times.

international investment treaty

It is discouraged now by some legal writers. In any event, it just means will or imposes an obligation, depending on context, and here is a technical legal discussion about that, which is not super relevant here:

shall, will and consistent legal writing [French documents usually use the present tense even when expressing obligation]

Everyday language: Traditionally, an intention to do something was usually expressed by: shall and will, shall for the 1st person singular and plural, and will for the other person**s. And in that sense, will can replace all shalls.

Please note: The Brits do tend to use it quite a bit this way, and in general, "Shall I" or "Shall we"" is used to offer to do something for someone in both "BrE" and "AmE".

• I shall [intend to leave] leave at 2 o'clock today unlike other days. They will leave [intend to leave] at 5 o'clock. [future in French and has the same feel of formality.]

• Shall I leave the door open when I leave? [Yes, please do, No, etc.] [Voulez-vous que je laisee, etc. pour rendre service]

This last use of shall to offer to do something is very idiomatic and the answers to it vary and are also idiomatic:

• Shall we go now? Yes, let's do.
• Shall I open the window? Yes, please do.

In AmE, will has mostly replaced any use of shall in everyday conversation (except the usage given above). But not necessarily in formal contexts. ["Shall we leave it at that?" Also, the contracted forms are very used: You'll, we'll, etc. "Shall" is no longer used much to express an obligation in everyday speech:

• You shall arrive on time every day. [that would be an order and is old-fashioned]

Summary:

-Must expresses an obligation in everyday usage.

• shall is a special case in legal language. (consistency rule: shall versus will)

• shall is either first person singular/plural for future intention in everyday usage: I shall do that work later.) Often used to offer to do something for someone or to make a suggestion about a situation: Shall we leave now? where "will" cannot be used. That use of shall in French is future or expressed as a future.

In everyday language, there should be no reason to confuse the use of must (obligation) and shall (suggestion or offer to do something, used in the first person singular and plural, mostly as a question).

• You must leave now! = Il faut que tu partes maintenant.
• Shall I open the window? = Voulez-vous que j'ouvre la fenêtre?
• Shall we go? = Partons(-nous)?
• You shall do it now. [désuet, en anglais] = Tu le finiras.
• You shall do it now. an order, in modern speech has become: You will do it now.

The only confusion between the two might possibly arise in legal language because in everyday language, the old-fashioned use of shall as in: "I say he shall go tomorrow"!, an order, which is the same as: I say he must go tomorrow!, is no longer commonly used in English. Even when expressing intention, will has mostly replaced shall ("I shall do it tomorrow" has become "I'll do it tomorrow") except for certain speakers who seem to love their "I shalls": "We shall finish this discussion in the morning. It's late." And that in French is a future tense.

Answer 4

English has a lot of inherent ambiguity, context matters a lot, and there may not be one single correct answer to a question. In general speech, you will need to rely on context to determine what's meant.

In the case of an official document such as an RFC, legal contract, or policies and procedures, and so forth, these terms are often defined in the documents themselves or in related documents, and in most cases, SHALL means the same thing as MUST, a mandatory requirement. This is a formal rule for a specific type of writing, not a general English language rule.

For example, in ISO standards writing, SHALL means a mandatory requirement. ANSI standards also use SHALL for mandatory requirements, but actually goes further and states that MUST is unacceptable, and only SHALL should be used.

In short, for specific business and legal documents, you need to know the specific definitions used in those contexts in order to determine what is meant.

It's sort of like programming, where a common word is assigned a very specific meaning in that context which is different from the usual meaning in spoken or written English.

Answer 5

Rather than duplicate some of the other detailed answers, a word swap illustration may help.

Compare the memorable quote "You shall not pass!" to the lamer "You must not pass!"

"Must" is weaker here, and implies the speaker's logically valid belief that it would be undesirable or unfortunate or impossible for the addressee to proceed further. It doesn't necessarily imply that the speaker would, or even can, do anything.

"Shall" is stronger, (or strongest, no other single English word is stronger here), and implies not just the speaker's logically valid belief, but the speaker's potential resolve to see that belief through. It suggests that if the addressee won't stop or fall back, then there will be a sufficient opposition either by nature or circumstance, perhaps guided or directed by some action of the speaker or the speaker's corporeal or supernatural allies.

(In practice, "shall" is more often spoken with more volume or force than "must".)

Answer 6

As a native English speaker, my understanding of the word "shall" in non-question conversational statements is that it indicates an expression of intent or strong predictive expectation. I do not recall ever seeing it defined as imposing a requirement or obligation. In any case, it is rarely used.

"Must" in most cases either creates or states the existence of a requirement or obligation. Whether the requirement is newly created or pre-existing depends on context. "Must" can also be used to emphasize a strong recommendation.

Answer 7

The difference outside of formal requirement-specification or legal environments is subtle, but that subtlety carries significance.

In order to drive a car you must start the engine, and you must take the car out of park and put it in gear.

If you ignore those, it really is impossible to drive a car. You might be able to roll down a hill, but you won't be able to go up hills, and you really won't be able to get anywhere you want to go unless "where you want to go" happens to be the bottom of the hill.

Once you have started the engine and taken the car out of park, there are still some "musts", e.g. for gasoline engines you must periodically add fuel, for electric engines you must have charged batteries, but now there are also a bunch of "shalls" that have zero meaning if the car is not started and taken out of park, but are critical unless you really want to have accidents or really want to get into legal trouble.

Unless you are "off-roading", you --

• shall drive on the proper side of the road.
• shall follow the laws of the road.
• shall take defensive action when a collision appears about to happen.

Lots of others. They really are not "musts" because you can drive a car without doing them, but if you do, bad things might happen, none of which will happen without starting the car and taking it out of park and putting it in gear.

Answer 8

No, there is no difference.

I will not go into generic everyday language issues, or the usage in other fields like laws, but from both the aspect of the RFC you quoted (which deftly defines SHALL and REQUIRED as equal to MUST), as well as practical use in the IT world in my experience, SHALL/MUST/REQUIRED mean exactly the same. They all mean that a sentence using these words specifies an absolute requirement.

Looking at that RFC, this is not the only case where multiple words are defined to be the same: SHOULD vs RECOMMEND, MAY vs OPTIONAL. MUST/SHALL is the only case where two verbs are specified with the same meaning - presumably because there were plenty of RFCs before the creation of RFC2119 which used those words interchangeably. The acknowledge this in chapter 8 of the RFC:

The definitions of these terms are an amalgam of definitions taken from a number of RFCs.

Answer 9

"You must not do this" implies that something should not be done because of bad consequences. "You shall not do this" implies that something should not be done because of the speaker's authority. There is some overlap but still a distinctive difference in meaning.

The KJV bible's commandment section is full of "thou shalt" and "thou shalt not" because it does not convey rules but commandments. While a modern reading of a lot of kosher/non-kosher distinctions and commandments conveys hygienic and health reasons, the science of the day was not sufficient for formulating a hard cause/consequence relationship. The commandments answer the "but why?" question essentially with "because I say so". Hence the use of "shalt/shalt not".

When the subject is first person ("I"/"we"), the difference is more marked: "I shall do this" strongly conveys a personal intent to go through with some act while "I must do this" conveys feeling compelled to go through with it.

Answer 10

In everyday English, I’d be more likely to use must if someone is being compelled to do something by force. I often use it when it’s not me who’s making the other person do something. “You must pay your taxes,” is a statement that you have no choice, but I’m not necessarily saying that I will be the one to make you do it, or that I approve. It’s common to follow a “must” clause with an “or” clause, stating the consequence of disobeying: “You must pay your taxes, or you will go to jail.”

Shall reads to me as a formal way of making a rule that I expect to be followed. “All employees shall submit expense reports,” is me making or invoking a rule, but not saying anything about enforcement. I wouldn’t use shall to make a specific request or tell one person what to do one time; shall is used for policies or rules.

I more often give a direct order with will, although, depending on context, that might mean something different. “You will send me a copy,” could mean that I’m ordering you, confirming what you agreed to do, or merely predicting your actions.

In technical writing, these words are used differently. Other answers have covered that very well. Even though the usages overlap, you would use must for requirements that are forced by some external constraint. So, for example, “All eight-bit unsigned values must be between 0 and 255.” The word shall would not make sense there, because the rule is impossible to break. On the other hand, “This value shall be between 0 and 100,” implies that it’s a choice I made. “This value must be between 0 and 100,” is something I’d be more likely to say if bad things would happen otherwise: “The value must be between 0 and 100, or the result of the calculation will be out of range.”

In nineteenth-century English grammar, which you’ll still find in some old books, there was a more complicated rule. The future tense was formed with the auxiliary verb, “I shall, we shall, you will, she/he/it will, they will.” Switching this, by saying, “I will” or “You shall” was in theory more emphatic, and expressed my determination to make something happen. Formal writing sometimes went along with this, but that was not how people spoke. There was a joke in the late 1800s that a grammarian was walking by a lake, and heard a young boy shout, “Oh no! I will drown, and no one shall save me!” The grammarian said, “Very well then, have it your way,” and walked off.

Answer 11

The differences are only in emphasis; nominally both must and shall express a mandatory rule, as the definition in RFC2119 says.

Traditionally, shall implied the power to define reality. An emperor would not simply utter a wish or an order; he would describe the future reality, because he had absolute power. For example, the Mexican emperor's ruling stated "the sentence shall be executed within twenty-four hours". The only answer was "consider it done".

In modern times such a decree is issued by a judge or, as here, by a technical regulation. Neither can, to the same degree, define reality; but they have uncontested power to define regulations.

Therefore, in modern use the emphasis of shall is that it creates a requirement for compliance with a regulation. For example, in the C++ standard draft we find the sentence "a conforming implementation shall [...] accept and correctly execute that program." The C++ committee issues a decree, very much like an emperor: It defines a future reality, in its limited domain. Each conforming implementation shall (as in a future action) do that because if it didn't, it would not be conformant. It's a little less clear in legalese like "both parties shall try to settle the dispute": This is the behavior defined by decree in this contract. If they don't, they are in violation (not conformant, so to speak).

Interestingly, because the modern institutions, including ISO standardization committies, are democratic by nature, this modern use of shall is a bit more of an agreement than a simple must. A contract has been signed by both parties. A C++ implementation author decided that it should be conformant, etc.

Must, by contrast, is neutral: It can be mandatory for any number of reasons, including forces of nature. It is not necessarily a decree.