Mandatory and directory words determine statutory obligations and permissive powers based on legislative intent and context. Ordinarily the words 'shall' and 'must' are treated as mandatory and 'may' as directory, but the statute construed as a whole may require the opposite meaning where literal application leads to absurdity or contradicts legislative intent. Words concerning public rights, the public good, or third party claims may justify giving directory words mandatory effect; prohibitory or negative terms are ordinarily mandatory. Clear evidence of contrary intent is required before departing from the words' usual meaning.