ruler vs foot rule

ruler and foot rule both are nouns.

Word	Noun	Adjective	Verb	Adverb
ruler	Yes	No	No	No
foot rule	Yes	No	No	No

As nouns, foot rule is a hyponym of ruler; that is, foot rule 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
foot rule: a ruler one foot long

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

ruler (noun)	foot rule (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 one foot long
a person who rules or commands