In a legal document, “shall” and “must” are both meant to create a binding obligation, but courts have interpreted “shall” to mean everything from “is required to” to “is permitted to,” while “must” is read almost universally as an absolute requirement. That is the practical difference between shall vs must in legal writing, and it is why federal rulemakers, plain-language advocates, and most modern drafting guides now recommend “must” whenever you want to impose a duty you can actually enforce.
What “Shall” Is Supposed to Do
“Shall” has been the standard word for creating a binding obligation in legal writing for centuries. When a lease says “the tenant shall pay rent on the first of each month,” the drafter intends to create a duty the tenant cannot ignore. The word is meant to operate as an imperative command, signaling that the action is mandatory rather than optional.1Legal Information Institute. Wex – Shall If the tenant doesn’t pay, the landlord can pursue remedies for breach.
One useful test is to substitute “has a duty to” for “shall.” If the sentence still makes sense, the word is being used correctly. If it doesn’t, the drafter has misused it. The most common misuse is attaching “shall” to something that isn’t a person. “Notice shall be given within 10 days” sounds authoritative, but notice isn’t a person and can’t carry a duty. The correct version names who has to act: “The buyer shall give notice within 10 days.” This error is everywhere in older contracts, and it is one reason the word has earned a reputation for creating confusion instead of preventing it.
What “Must” Does
“Must” establishes an absolute requirement with almost no interpretive wiggle room. When a filing deadline says a document “must be submitted by March 15,” no one reading that sentence wonders whether the date is a suggestion. The word does one thing: it commands.
“Must” also works cleanly for conditions precedent, the steps you have to complete before a right kicks in. A regulation might say an applicant “must submit a background check before receiving a license.” The sequence is obvious: no background check, no license. For prohibitions, “must not” is the clearest way to say something is forbidden. Older documents often use “shall not,” but that carries the same ambiguity problems as “shall” by itself.
Why “Shall” Creates Ambiguity
Legal scholars call “shall” a chameleon word because its meaning shifts with context, and courts have never settled on a single reading. As one legal-writing authority put it, “shall has become so corrupted by misuse that it has no firm meaning. It can mean ‘must,’ ‘should,’ ‘will,’ ‘may,’ or ‘is.'”2Illinois State Bar Association. Legal-writing tip: Must vs. shall Courts in virtually every English-speaking jurisdiction have at some point held that “shall” means “may,” and vice versa.
The U.S. Supreme Court addressed this directly in Gutierrez de Martinez v. Lamagno, where Justice Ginsburg’s opinion acknowledged that although “shall” generally means “must,” legal writers sometimes use or misuse it to mean “should,” “will,” or even “may,” and pointed to Federal Rules that used “shall” merely to authorize rather than require action.3Legal Information Institute. Gutierrez de Martinez v. Lamagno, 515 U.S. 417 (1995) When the Supreme Court openly treats a word as unreliable, that tells you something about the drafting risk.
Mandatory Versus Directory Readings
The most consequential split is whether a court reads “shall” as mandatory or merely directory. A mandatory reading means failure to comply invalidates the action or triggers penalties. A directory reading treats the provision as a procedural guideline, so noncompliance voids nothing and the party who ignored it faces no real consequence. Illinois courts, for example, presume that a “shall” directed at a government official is directory rather than mandatory unless the statute specifies a consequence for noncompliance or a directory reading would injure the right the provision was designed to protect.4Illinois State Bar Association. When the term shall is directory
This is where real money gets lost. If you are relying on a contract clause that says the other party “shall” do something, and a court decides the provision is merely directory, the other side may escape consequences for ignoring it. You thought you had a binding obligation. The court says you had a suggestion. Replacing “shall” with “must” does not guarantee a mandatory reading in every case, but it removes the most common argument for a directory one.
The Cost of Litigating a Single Word
Disputes over whether “shall” means “must” or “may” generate real litigation expense. When a provision’s meaning is unclear, both sides end up arguing about a word instead of the substance of their disagreement. The drafter assumed “shall” was ironclad, the opposing lawyer found a case where a court read it as permissive, and now both parties are spending money on a question that better drafting would have prevented.
Where Each Word Falls on the Spectrum
The shall-vs-must choice makes more sense when you see it next to the other words drafters reach for. From strongest obligation to pure discretion:
- Must — an absolute requirement. No discretion and no exceptions unless the document creates one. This is the modern standard for imposing a duty.
- Shall — intended as a mandatory duty but read inconsistently by courts. Carries higher litigation risk than “must.”
- Will — creates a contractual promise or describes a future event. Some drafters use it to impose obligations, but a court may read it as predicting what will happen rather than commanding it.
- Should — a recommendation or expectation, not a binding requirement. Noncompliance rarely triggers penalties.
- May — grants permission or discretion. Courts ordinarily treat “may” as permissive, letting the party choose whether to act. When a single statute uses both “may” and “shall,” courts are more likely to read “shall” as mandatory, since the drafter presumably chose different words for different reasons.5Legal Information Institute. Wex – May
A well-drafted document uses “must” for requirements, “must not” for prohibitions, “may” for discretion, and “should” for recommendations. That four-word system covers almost every situation a drafter encounters and leaves very little for a court to argue about.
Why the Federal Government Dropped “Shall”
The federal government has been systematically removing “shall” from its rules for over two decades. The Federal Rules of Civil Procedure were restyled in 2007, and every instance of “shall” was replaced with “must,” “may,” or “should,” depending on context.6Office of the Law Revision Counsel. Federal Rules of Civil Procedure – Rule 1 The Federal Rules of Criminal Procedure, Appellate Procedure, and Evidence went through the same treatment. The advisory committee for the Rules of Evidence put the reason bluntly: “The word ‘shall’ can mean ‘must,’ ‘may,’ or something else, depending on context. The potential for confusion is exacerbated by the fact the word ‘shall’ is no longer generally used in spoken or clearly written English.”7Office of the Law Revision Counsel. 28 USC App, Federal Rules of Evidence, Article I
Congress reinforced the direction with the Plain Writing Act of 2010, which requires federal agencies to use plain language in documents that explain how to obtain benefits, file taxes, or comply with federal requirements.8GovInfo. Public Law 111-274, Plain Writing Act of 2010 The guidelines implementing the Act instruct agencies to use “must” instead of “shall” when creating requirements.
Practical Choices When Drafting or Reviewing
If You Are Writing a New Document
Use “must” for every requirement, “must not” for every prohibition, and “may” wherever the party has a genuine choice. Reserve “will” for statements about future events or consequences, such as “if the buyer defaults, the seller will have the right to terminate,” rather than for imposing duties. Keep the “has a duty to” substitution test in your back pocket. Any time you write “must,” swap in “has a duty to.” If the sentence sounds wrong, the subject usually isn’t someone capable of having a duty. Fix the subject, then use “must.”
If the Document Already Uses “Shall”
You can’t always rewrite an entire contract. One option is to add a definitions or interpretive clause stating that every use of “shall” in the agreement is intended as mandatory and imperative, not permissive or directory. This won’t fix every ambiguity, but it gives a court clear evidence of intent if a dispute arises.
When reviewing a contract someone else drafted, pay close attention to “shall” provisions that affect your obligations or deadlines. If a clause says you “shall” do something and you would face real consequences for falling short, ask for an amendment replacing “shall” with “must.” The other party will rarely object, because if the provision was always meant to be mandatory, the word change doesn’t alter the deal. If they do object, that tells you something important about what they think the clause actually requires.
If You Are Reading a Statute or Regulation
When you encounter “shall” in a statute, don’t assume it means “must.” Look at the surrounding text. If the statute specifies a consequence for noncompliance, the provision is almost certainly mandatory regardless of which word is used. If the statute uses both “shall” and “may” in different provisions, courts are more likely to treat the “shall” provisions as mandatory, since the drafter chose different words deliberately.5Legal Information Institute. Wex – May And if the “shall” is directed at a government official with no stated penalty, there is a reasonable chance a court will read it as directory rather than mandatory.