There is a class of words having always an adjectival use in general, but with such subtle functions and various meanings that they deserve separate treatment. In the sentence, "He passes an ordinary brick house on the road, with an ordinary little garden," the words the and an belong to nouns, just as adjectives do; but they cannot be accurately placed under any class of adjectives. They are nearest to demonstrative and numeral adjectives.
The article the comes from an old demonstrative adjective (sē, sēo, ðat, later thē, thēo, that) which was also an article in Old English. In Middle English the became an article, and that remained a demonstrative adjective.
An or a came from the old numeral ān, meaning one.
Our expressions the one, the other, were formerly that one, that other; the latter is still preserved in the expression, in vulgar English, the tother. Not only this is kept in the Scotch dialect, but the former is used, these occurring as the tane, the tother, or the tane, the tither; for example,—
We ca' her sometimes the tane, sometimes the tother.—Scott.
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