Legal? Well, maybe not for carry-on, etc. but my Merchant Marine days made me say "Don't leave home without it":

as well as a bigger, sturdier buck knife, just for line(as in rope)-cutting emergencies, etc. Seamen on deck w/o knives got chewed out.

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Give sorrow words: the grief that does not speak
Whispers the o'er-fraught heart and bids it break.

-William Shakespeare, Macbeth, Act IV, Sc. III