flat knot vs square knot

flat knot and square knot both are nouns.

Word	Noun	Adjective	Verb	Adverb
flat knot	Yes	No	No	No
square knot	Yes	No	No	No

As nouns, square knot is a hypernym of flat knot; that is, square knot is a word with a broader meaning than flat knot:

flat knot: a square knot used in a reef line
square knot: a double knot made of two half hitches and used to join the ends of two cords

flat knot (noun)	square knot (noun)
a square knot used in a reef line	a double knot made of two half hitches and used to join the ends of two cords