ruler vs yardstick

ruler and yardstick both are nouns.

Word	Noun	Adjective	Verb	Adverb
ruler	Yes	No	No	No
yardstick	Yes	No	No	No

As nouns, yardstick is a hyponym of ruler; that is, yardstick is a word with a more specific, narrower meaning than ruler:

ruler: measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths
yardstick: a ruler or tape that is three feet long

Other hyponyms of ruler include carpenter's rule, foot rule, meterstick, metrestick, yard measure.

ruler (noun)	yardstick (noun)
measuring stick consisting of a strip of wood or metal or plastic with a straight edge that is used for drawing straight lines and measuring lengths	a ruler or tape that is three feet long
a person who rules or commands	a measure or standard used for comparison