Using Must, Must Not, and May
Open Course Commonwealth Legislative Drafting

CRLP Guidance and Training for Drafters / Lesson

Using Must, Must Not, and May

Choose modal wording by matching it to the legal effect, the bearers of the rule, and the consequence of exercising or not exercising the provision.

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Part of CRLP Guidance and Training for Drafters

In this lesson

  • use must and must not for true obligations and prohibitions
  • distinguish permission from power when using may
  • avoid ambiguous may not and check the consequence structure behind each modal choice

Modal wording should follow legal structure, not habit.

Use must and must not for true conduct rules

Must and must not belong to provisions which regulate what a person or body is required or forbidden to do.

That normally means:

  • a real bearer of the rule
  • a clear act or omission
  • a consequence if the rule is broken

Sometimes that consequence is express, such as an offence or penalty. Sometimes it is supplied elsewhere, such as invalidity, dismissal, loss of entitlement, or public law challenge. Either way, the drafter should know what the consequence is.

A must-clause carries a breach structure

Once the law says a person must do something, the law also creates the possibility that the person does not do it.

That is why every must clause should be checked against the question what if not. If the breach proposition has no intelligible consequence, the drafter may need a different structure.

Some must-clauses are only superficially normative

A filing deadline or form requirement may look like a duty while really functioning as a gateway to another rule.

That does not make must automatically wrong, but it does mean the drafter should test whether the legal consequence would be clearer if stated more directly.

May can mean permission or power

May does different work from must.

It does not create a breach in the same way. Instead, it leaves room for choice or gives a legal capacity to act.

Two common uses are:

  • permission: the person is allowed to do something that might otherwise be restricted
  • power: a person or body may take a step that changes another person’s legal position or triggers another rule

Those uses should not be collapsed into one vague idea of optionality. The drafter should know whether the provision is creating freedom, conferring authority, or both.

May still needs consequence analysis

Even though may is not contravened in the same way, it still requires consequence thinking.

Ask:

  • what happens if the person exercises the permission or power?
  • what happens if the person does not?
  • which linked provision gives legal effect to that choice?

That is especially important when a power matters only because another provision makes something happen once the power is exercised.

Avoid ambiguous may not

May not is risky because the not may attach either to the modal or to the action.

It can mean:

  • prohibition: the person is not permitted to do the act
  • permission not to act: the person is free to refrain

Those are not the same. When the intended meaning is prohibition, use must not. When the intended meaning is freedom not to act, say so directly or use need not where that is the better fit.

Self-check

Use these prompts to test whether the drafting point is clear before you move on.

1What makes a “must” clause more than a style choice?

It creates a rule that can be broken, so the drafter should be able to identify the bearer of the duty, the relevant conduct, and the consequence of non-compliance.

2Why is “may not” often best avoided?

Because it can mean either prohibition or permission not to act. Using clearer alternatives avoids that ambiguity.