Examples and Observations:
"All the words in a language can be broadly divided into two categories, open and closed. The closed category is so called because it does not easily accept new words. Its members are fixed and do not usually change . . .. The open category . . . contains nouns, verbs, adverbs, and descriptive adjectives--exactly those parts of speech that remain open to new additions . . ..
"Words in the open category are usually further divided into simple and complex words. Simple words contain just one morpheme (house, for example, or walk, slow, or green), whereas complex words contain more than one (houses, walking, slowly, or greenest)."
(Thomas E. Murray, The Structure of English. Allyn and Bacon, 1995)