In legal writing, “must” and “shall” were both meant to create binding obligations, but courts have interpreted “shall” so inconsistently that modern drafters use “must” for requirements and reserve “shall” for almost nothing at all. The U.S. Supreme Court has acknowledged that legal writers sometimes use “shall” to mean “should,” “will,” or even “may,” which makes it unreliable as a command word.1Cornell Law Institute. Gutierrez de Martinez v. Lamagno, 515 U.S. 417 (1995) The federal courts replaced “shall” with “must” throughout the Federal Rules of Civil Procedure in 2007, and federal agencies have followed in their own writing standards.2Cornell Law Institute. Federal Rules of Evidence Rule 101 – Section: Changes to Reduce Inconsistent, Ambiguous, Redundant, Repetitive, or Archaic Words
Why “Shall” Lost Its Precision
For centuries, “shall” was the standard word for creating a binding duty. A lease that read “the tenant shall pay rent on the first of each month” was understood to leave the tenant no choice. Drafters of statutes, wills, and commercial agreements used it the same way, treating it as a verbal command.
The trouble was overuse without discipline. Writers started dropping “shall” into sentences where they actually meant “will” (a prediction about the future), “should” (a recommendation), or “may” (a permission). A single contract might use “shall” in three different senses across five pages, and the drafter might not notice. The federal restyling committee observed that “shall” can mean “must,” “may,” or something else depending on context, and that the word is no longer in common spoken or written English, which deepens the confusion.2Cornell Law Institute. Federal Rules of Evidence Rule 101 – Section: Changes to Reduce Inconsistent, Ambiguous, Redundant, Repetitive, or Archaic Words
How Courts Have Read “Shall” Both Ways
The real damage shows up in courtrooms. When parties disagree about what a statute or contract requires, judges have to decide what “shall” meant in that specific sentence, and the results are unpredictable. In Gutierrez de Martinez v. Lamagno, the Supreme Court acknowledged that courts in virtually every English-speaking jurisdiction have held that “shall” can mean “may” in certain contexts, and vice versa.1Cornell Law Institute. Gutierrez de Martinez v. Lamagno, 515 U.S. 417 (1995) The Court pointed to federal rules that used “shall” not to require action, but merely to authorize it.
Some courts have treated “shall” as mandatory, meaning noncompliance voids a right or triggers a penalty. Others have read it as directory, meaning the action was expected but failing to do it carried no automatic consequence. When the same word can mean opposite things in different courtrooms, the word has stopped doing its job.
The Federal Shift to “Must”
The federal government addressed the problem through two reforms: restyling its court rules and passing the Plain Writing Act of 2010.
The 2007 Restyling of Federal Court Rules
In 2007, the Federal Rules of Civil Procedure went through a comprehensive restyling. The drafting committee systematically replaced “shall” with “must,” “may,” or “should,” choosing whichever word matched the established interpretation of each rule.2Cornell Law Institute. Federal Rules of Evidence Rule 101 – Section: Changes to Reduce Inconsistent, Ambiguous, Redundant, Repetitive, or Archaic Words The Federal Rules of Evidence later followed the same approach. The changes were intended to be purely stylistic, not to alter any legal rights, but the message was clear: “shall” was too unreliable to keep using.
The committee’s reasoning was straightforward. Because “shall” had accumulated so many meanings, every use of it forced the reader to guess which meaning the drafter intended. Replacing it with “must” for requirements, “may” for permissions, and “should” for recommendations eliminated the guesswork.
The Plain Writing Act and Agency Guidance
Congress reinforced the shift in 2010 with the Plain Writing Act, which requires every federal agency to use clear, concise language in documents that explain government benefits, services, or compliance requirements.3GovInfo. Public Law 111-274 Plain Writing Act of 2010 The Act doesn’t ban “shall” outright, but the federal writing guidelines it produced are explicit: use “must” for requirements.
Individual agencies have gone further. The FAA’s writing standards tell drafters to avoid “shall” entirely, calling it ambiguous because it can mean “must,” “ought,” or “will,” and noting that writers have incorrectly used it to mean “should” or “may.”4Federal Aviation Administration. FAA Order 1000.36 FAA Writing Standards The Department of Defense’s Writing Style Guide uses “must” as its standard for mandatory language throughout official issuances.5Department of Defense. Writing Style Guide and Preferred Usage for DoD Issuances
The Modern Set of Legal Verbs
Current drafting guidelines have settled on a clean set of words, each carrying a distinct level of obligation.
- Must creates an absolute requirement. “The borrower must submit proof of insurance before closing” means there is no discretion.
- Must not creates an absolute prohibition. “The tenant must not sublease the unit” bars subleasing entirely. This is the preferred form because “may not” can sometimes read as a prediction rather than a restriction.
- Should is a strong recommendation that falls short of a requirement. “The applicant should include supporting documentation” means it’s a good idea and might cause problems if skipped, but it isn’t grounds for automatic rejection.
- May grants permission or discretion. “The court may extend the deadline” means the court has the power to do so but isn’t required to. Courts generally treat “may” as permissive, particularly when the same statute also uses “must” or “shall” as a mandatory term, since the drafter presumably chose different words for different meanings.6Legal Information Institute. May
- Will describes future action or states intent. “The company will deliver the goods by June 1” reads as a promise or prediction. Plain-language guidance keeps “will” for future-tense statements and uses “must” when the sentence creates a legal obligation.
The clean rule: “must” for obligations, “must not” for prohibitions, “may” for permissions, “should” for recommendations, “will” for statements about the future.
What Happens When “Must” Is Ignored
Mandatory language does more than signal intent. It creates enforceable duties with real consequences, and the specific fallout depends on whether the language sits in a court rule or a contract.
In Court Rules
Federal court rules use “must” to impose duties on both the court and the parties. Under Federal Rule of Civil Procedure 37, if a court grants a motion to compel discovery, it must require the noncompliant party to pay the other side’s reasonable expenses, including attorney’s fees.7Legal Information Institute. Rule 37 Failure to Make Disclosures or to Cooperate in Discovery Sanctions The court “must” impose fees, not “may.” The judge has no discretion to skip that step.
For more serious discovery violations, courts can treat disputed facts as proven against the noncompliant party, bar them from presenting evidence, strike their pleadings, or enter a default judgment. Evasive or incomplete responses are treated the same as outright refusals.7Legal Information Institute. Rule 37 Failure to Make Disclosures or to Cooperate in Discovery Sanctions
In Contracts
When a contract says a party “must” do something before receiving something in return, the language often creates a condition precedent: an event that has to happen before the other side’s obligation starts. If a real estate contract says the buyer must obtain financing by a certain date, missing that date doesn’t just breach the contract. It means the seller’s duty to close never arises in the first place.
The practical difference matters. A regular breach gives the injured party a right to sue for damages. Failure to satisfy a condition precedent leaves nothing to sue over because the other party’s obligation never started. Anyone trying to enforce the contract bears the burden of proving that all conditions were met or properly excused. Missing a “must” deadline can mean losing the deal entirely, not just paying damages.
Reading Older Statutes and Contracts That Still Use “Shall”
The shift to “must” covers the restyled federal rules, modern agency guidance, and new contracts drafted by careful attorneys. It doesn’t cover the enormous body of existing law. Thousands of federal and state statutes still use “shall” because they were written before the restyling movement, and many private contracts follow older templates.
When you encounter “shall” in a statute or contract today, context controls. Courts will look at the surrounding language, the document’s purpose, and how other courts have read the same provision. If the same document uses both “shall” and “may,” a court is more likely to treat “shall” as mandatory because the drafter apparently chose different words on purpose.6Legal Information Institute. May If the document uses “shall” loosely throughout, the outcome is harder to predict.
For anyone drafting new, the practical advice is simple. Don’t use “shall.” Every federal writing standard, every major style guide, and every plain-language reform of the last two decades points the same way. Use “must” when you mean to create a requirement, and save yourself the argument about what “shall” was supposed to mean.