Allen and Greenough's New Latin Grammar, section 235

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Significant Endings.

235.

Both primary and secondary suffixes, especially in the form of compound suffixes, were used in Latin with more or less consciousness of their meaning. They may therefore be called Significant Endings.

They form: (1) Nouns of Agency; (2) Abstract Nouns (including Names of Actions); (3) Adjectives (active or passive).

NOTE: There is really no difference in etymology between an adjective and a noun, except that some formations are habitually used as adjectives and others as nouns (§ 20. b. N. 2).