two-by-four vs timber

two-by-four and timber both are nouns.

Word	Noun	Adjective	Verb	Adverb
two-by-four	Yes	No	No	No
timber	Yes	No	No	No

As nouns, timber is a hypernym of two-by-four; that is, timber is a word with a broader meaning than two-by-four:

two-by-four: a timber measuring (slightly under) 2 inches by 4 inches in cross section
timber: a beam made of wood

two-by-four (noun)	timber (noun)
a timber measuring (slightly under) 2 inches by 4 inches in cross section	a beam made of wood
	a post made of wood
	(music) the distinctive property of a complex sound (a voice or noise or musical sound)
	land that is covered with trees and shrubs
	the wood of trees cut and prepared for use as building material