foot rule vs ruler

foot rule and ruler both are nouns.

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

As nouns, ruler is a hypernym of foot rule; that is, ruler is a word with a broader meaning than foot rule:

foot rule: a ruler one foot long
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

Other hypernyms of foot rule include rule.

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