What is the difference between shall and must

Look at Figure I. Therefore, you should attempt to reach the lower temperature in one minute. The easiest way to do this is to have a cold water injection ready to use — open a valve and inject the cold water. Close the valve and return to using hot water. Yes, this can be a thermal shock to the seal — that was the intent.

Buck; When working in oil refining, the company I worked at defined the term Shall as it had to be adhered to. Should meant it still had to be adhered to, but if it was not adhered to, an explanation in writing was required. Whether or not the API interpretation and definition would hold up in a court of law is another question.

Some facilities which have been set up to do this type of testing cannot drop the hot oil temperture in a quick manner, some take about hours in order to cool down enough to meet the test parameters per the specification. This means that the full cyclic phases take days to compete. Others which I am familiar with set up an addition loop of cool test fluid which could be switched over quickly and the 5 cyclic phases could be completed in less than a few hours. Must is used informally.

Both the words shall and must have similar meanings but are to be used in a sentence in a very different context, or else the meaning of the sentence would entirely change, leading to miscommunication. Found in old English form. This word is a modal verb used in the place of the first, second, and third-person pronoun. It subjects a sentence leading to future tense. This word is a modal auxiliary verb. It must come first in a verb phrase after the subject and before the second verb.

Generally, this word is used to express ideas and rules and when a speaker wishes to convey something that will take place in the future. This word is generally used to emphasize the need for a particular requirement in a sentence that has to be done.

Sometimes used in past tense too. By contrast, the Alberta Law Review noted in 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. 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:. 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:.

There may of course be exceptions. 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. 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. 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! 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. As found in orders or instructions , in formal language, including technical language. 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. 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:. 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:.

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. Shall I leave the door open when I leave? This last use of shall to offer to do something is very idiomatic and the answers to it vary and are also idiomatic:.

In AmE, will has mostly replaced any use of shall in everyday conversation except the usage given above. But not necessarily in formal contexts. Often used to offer to do something for someone or to make a suggestion about a situation: Shall we leave now? 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.

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 "! 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. 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. 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.

Compare the memorable quote "You shall not pass! It doesn't necessarily imply that the speaker would, or even can, do anything. 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". 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. Whether the requirement is newly created or pre-existing depends on context. 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. 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.

They all mean that a sentence using these words specifies an absolute requirement. The acknowledge this in chapter 8 of the RFC:. There is some overlap but still a distinctive difference in meaning.