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In some senses, mandatory is marked as dated, rare. Watch for register when choosing this word.
ADJ.
environmental, minimum
All new cars must meet mandatory environmental standards before they can be sold.
VERB + MANDATORY
belts, faces, isn't, level, reform, regard, say, stops
Wearing seatbelts is mandatory for all passengers in every vehicle.
MANDATORY + NOUN
action, children, disclosure, forbidden', life, schools
Wearing seatbelts in cars became mandatory after new safety laws were passed.
PREP.
in, on, with
Wearing a seatbelt is mandatory in most countries around the world.
ADV.
only, usually
Wearing a seatbelt is usually mandatory in most countries around the world.