placard vs show card

placard and show card both are nouns.

Word	Noun	Adjective	Verb	Adverb
placard	Yes	No	Yes	No
show card	Yes	No	No	No

As nouns, show card is a hyponym of placard; that is, show card is a word with a more specific, narrower meaning than placard:

Other hyponyms of placard include show bill, theatrical poster, flash card, flashcard.

placard (noun)	show card (noun)
a sign posted in a public place as an advertisement	a poster advertising a show or play

placard (verb)	show card (verb)
publicize or announce by placards
post in a public place