5.1.1. In IE III, the noun could be determined in three different ways: with another noun, as in stone wall; with a noun in Genitive, as in the father's house; or with an adjective, as in paternal love. These are all possible in Europaio, but the adjective corresponds to the third way, i.e., to that kind of words, possibly derived from the Genitive, that are declined to make it agree in case, gender and number with the noun they define.
5.1.2. The adjective is from the older stages like a noun, and even today the Europaio-derived languages have the possibility to make an adjective a noun (as English), or a noun an adjective (stone wall). Furthermore, some words are nouns and adjectives as well: wrsen, male, man, can be the subject of a verb (i.e., a noun) and can determine a noun.
Most of the stems are actually indifferent to the opposition noun/adjective, and even most of the suffixes. The inflection is common, too; any difference is normally secondary. This is the reason why we have already studied the adjective declensions; they are the same as the noun's.
5.1.3. However, since the older IE there were nouns different from adjectives, as wlqos, pods,.. and adjectives different from nouns, as rudhros, solwos, etc. Nouns could, in turn, be used as adjectives, and adjectives be nominalized.
NOTE. There were already in IE II times some trends of adjective specialization, with the use of suffixes, vocalism, accent and sometimes inflection which changed a noun to an adjective and vice versa.