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27 March 2011

Must versus Shall

Updated 2 April 2011

Many think "shall" wishy-washy and "must" unambiguous. They have it backwards.

Few understand the differences between "will," "must," and "shall." For example, since "shall" conveys a sense of weightiness, people often use it pretentiously when "will" would do.

The ambiguity of "shall" lies not in the word, but in confused vocabularies. "Must" has ambiguous timing, certainty, and force whereas "will" and "shall" imply future fulfillment. Additionally, "shall" implies certainty and authority.

A "must" may occur at any time, and its importance can range from zero to critical. One could say "it must have been," "it must be," or "you must obey." "Must" can imply reasons ranging from fulfilling a desire ("You simply must visit us!"), achieving an ends ("To retire comfortably, you must save"), or avoiding injury ("You must remember your anniversary"), to fulfilling a requirement ("The roses must be red"). It can also denote a high probability ("Since A, B, and C are impossible, the answer must be D").

A "shall" may occur only in the future relative to the time of writing, and it implies not merely prediction as "will" does, but it implies certainty or determination ("It shall come to pass that locusts will devour your grain..."). Legal and contractual language assumes satisfaction of the requirement, so "shall" denotes a command or a requirement that will, with certainty, come to pass. I did not say, "shall, with certainty, come to pass" because that would have been redundant.

"Must" carries ambiguity regarding not only timing and certainty, but also regarding consequences. Another way to say this is that "must" leaves open the question of "why?" whereas "shall" makes it clear that this is a contractual requirement. One might say "you must," but saying "you shall" implies "or else...." Shall implies a future, required or commanded action or condition, with consequences if not fulfilled.

"Must" is ambiguous. "Shall" has a narrow meaning and usage -- for those who understand the terms, at least.

Part 2.

I agree with using simpler language. "Shall" only applies to requirements statement such as

Writers employed by ABCXYZ Company shall only use "shall" to indicate requirements.

Dumbing writing down just because people don't grasp the precision of the imperative "shall" (or where to use it) crosses the line.

Someone asked, "isn't it always up to the client?" The client may have a style guide over which the employee has no influence, so sometimes, it is up to the client's preferences.

However, it is not always "all up to the client." Clients have the final say, but the Business or Engineering experts hire writers for their expertise in technical English. Technical writers should dig for expertise in their domain and tell clients what they need and why they need it. Otherwise, they should hand over their jobs to admins.

In a discussion, someone challenged me with definitions of "must" and "shall." He also claims that an Illinois Supreme Court ruling, PEOPLE v. GARSTECKI, cast doubt on the clarity of "shall."


  1. plan to, intend to, or expect to: I shall go later.

  2. will have to, is determined to, or definitely will: You shall do it. He shall do it.

  3. (in laws, directives, etc.) must; is or are obliged to: The meetings of the council shall be public.

  4. (often in invitations): Shall we go?

  1. to be obliged or bound to by an imperative requirement: I must keep my word.

  2. to be under the necessity to; need to: Animals must eat to live.

  3. to be required or compelled to, as by the use or threat of force: You must obey the law.

  4. to be compelled to in order to fulfill some need or achieve an aim: We must hurry if we're to arrive on time.

  5. to be forced to, as by convention or the requirements of honesty: I must say, that is a lovely hat.

  6. to be or feel urged to; ought to: I must buy that book.

  7. to be reasonably expected to; is bound to: It must have stopped raining by now. She must be at least 60.

  8. to be inevitably certain to; be compelled by nature: Everyone must die.

Neither #1 definition applies because both refer to a first person context, and I only defend "shall" used in third person sentences that state mandatory requirements.

Definition 2 of "shall," "will have to, is determined to, or definitely will" indicates confident prediction. Although this is not the applicable definition, it still leaves no wiggle room for non performance, should the reader mistakenly use this meaning.

Definition 3 has two points. "In laws, directives, etc." is not limited to laws. The IEEE has trended toward use of "shall." That applies to "directives, etc." such as processes, statements of work, and requirements specifications. Engineers aren't known for their flawless grammar, but this does demonstrate that in at least some usages, "shall" has taken on an unambiguous meaning.

("Shall" Def. 3, cont'd) "Must; is or are obliged to: The meetings of the council shall be public" repeats the idea that no wiggle room remains for non performance. The Illinois Supreme Court ruling in People v. Garsteki, does cite a Third Court of Appeals ruling that "shall" sometimes means "may" or "must;" but then it clarifies with a Second Court of Appeals ruling that "'shall' had a directory (strong advisory) reading when it was modified by the phrase 'whenever practicable.'"

In other words, "shall" means "mandatory" until defined conditions negate it. The meaning "may" was not inherent in "shall" itself.

Moreover, the ruling that the Illinois Supreme Court reviewed pivoted not on the meaning of "shall," but on whether the lower court observed exceptions and conditions contained in the relevant stature. It had little bearing on "shall" versus "must."

Definition 2 of "must" -- the first definition applicable to third-person usage as in requirements documents -- "to be under the necessity to; need to: Animals must eat to live," implies reference to a cause.

"Shall" avoids the question, "why?" by asserting authority whereas "must" leaves open the possibility of mitigating the reason. That's a dangerous ambiguity for creative types.

A creative astrohusband might address the cause by putting his cows in hibernation or might address the need by feeding his pigs intravenously, thereby circumventing the "must." The first applicable definition of "must," therefore leaves wiggle room for non compliance.

"Must" definition 3, "to be required or compelled to, as by the use or threat of force," fulfills the need of a compliance document.

Definition 4, "to be compelled to in order to fulfill some need or achieve an aim," parallels definition 2. The sense is not "mandatory," but "needed," which could be addressed in other ways. "Must" demands the question, "why?" Any term that demands more questions than it solves cannot be called "unambiguous."

Definition 5, "to be forced to, as by convention or the requirements of honesty: I must say, that is a lovely hat," would fit right into the structure of a requirement statement. Why must the operator place the box on the top shelf? Because that's his duty? Because that's where we've always kept it? You can't exclude this meaning, just from the structure of the sentence.

I admit, "must" definition 6, "to be or feel urged to; ought to," is conversational. However, some argue against "shall" based on how people process it. This meaning of "must" works (incorrectly) in a typical requirement statement. Definition 1 of "shall" does not. That renders disambiguation of "must" much more difficult. The same applies to definitions 7, "to be reasonably expected to; is bound to," and 8, "to be inevitably certain to; be compelled by nature."

The two (2) applicable definitions of "shall" imply unambiguously the certainty of a requirement. The seven (7) applicable definitions of "must" imply weight that varies from desire to compulsion.

As Garstecki states, if in 3rd-person use, "shall" means anything other than that the verb is mandatory, then it is superfluous; if it is there, it is there for a reason.

But I could be wrong.

Additional reading from both sides of the issue:

PEOPLE v. GARSTECKI, Illinois Supreme Courte, No. 106714, September 24, 2009.

“Shall” Versus “Will.” Grammar Girl Episode 119, July 22, 2008.

The Judicial Council of California.

The US Food and Drug Administration.

Ministry of Economic Development (New Zealand)

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