Shall - a horrible little word, legally speaking.

The word 'shall' is often used in legal documentation. Or, I should say that the word 'shall' is often misused in legal documents.

'Shall' means 'have a duty', an obligation to do something. But in legal documents the real meaning of the word 'shall' can become confusing. In various court cases 'shall' has been held to mean: may, must, will and is (Source: Legal Writing in Plain English, Bryan A Garner).

When you draft a legal document, avoid 'shall'. Do not use this misleading word. Use, instead, 'must' for obligation and 'may' to indicate that someone can decide to act or not. Note that in contractual documents, 'will' is more often used than 'must' to express obligation. (Although 'must' is much clearer, in my considered opinion.)

You have been warned. You shall obey.