mainstay vs forestay

mainstay and forestay both are nouns.

Word	Noun	Adjective	Verb	Adverb
mainstay	Yes	No	No	No
forestay	Yes	No	No	No

As nouns, forestay is a hypernym of mainstay; that is, forestay is a word with a broader meaning than mainstay:

mainstay: the forestay that braces the mainmast
forestay: an adjustable stay from the foremast to the deck or bowsprit; controls the bending of the mast

mainstay (noun)	forestay (noun)
the forestay that braces the mainmast	an adjustable stay from the foremast to the deck or bowsprit; controls the bending of the mast
a central cohesive source of support and stability
a prominent supporter