In the Indo-European languages where a definite article exists, it is historically derived from a demonstrative pronoun. The hypothesis of this paper is that the origin of the definite article is the creation of noun phrase structure by the subordination of a noun to a demonstrative pronoun. This process is described for the Romance languages and Danish. In languages where an indefinite article exists, it is historically derived from the numeral ‘one’. This origin of the indefinite article points out two possible directions for its further evolution: it can continue as a quantifier or it can become a classifier. The quantifier road is illustrated by the plural of the indefinite article in Old French and Spanish, the classifier use by the two indefinite articles of Modern French. Definite and indefinite articles thus have different functions and values: definite articles are pronominal heads, indefinite articles are quantifiers or classifiers.