joist vs trimmer joist

joist and trimmer joist both are nouns.

Word	Noun	Adjective	Verb	Adverb
joist	Yes	No	No	No
trimmer joist	Yes	No	No	No

As nouns, trimmer joist is a hyponym of joist; that is, trimmer joist is a word with a more specific, narrower meaning than joist:

joist: beam used to support floors or roofs
trimmer joist: joist that receives the end of a header in floor or roof framing in order to leave an opening for a staircase or chimney etc.

Other hyponyms of joist include floor joist, trimmer.

joist (noun)	trimmer joist (noun)
beam used to support floors or roofs	joist that receives the end of a header in floor or roof framing in order to leave an opening for a staircase or chimney etc.