When two or more words are joined to make a longer word, the process is know as compound-formation. A compound word can be a noun(.....), an adjective (....), a verb (....), an adverb (.....) , a preposition (.....) a conjunction (....). The only word class in english which can never occur in the form of a compound word is the article. Of these different types of compound-formationsn english, compound nouns seem to be the largest in number and compound adjectives seem to be the second largest.