main deck vs second deck

main deck and second deck both are nouns.

Word	Noun	Adjective	Verb	Adverb
main deck	Yes	No	No	No
second deck	Yes	No	No	No

As nouns, main deck and second deck are synonyms defined as:

main deck and second deck: the uppermost sheltered deck that runs the entire length of a large vessel

main deck (noun)	second deck (noun)
the uppermost sheltered deck that runs the entire length of a large vessel	the uppermost sheltered deck that runs the entire length of a large vessel