Two real articles before a noun – why?

Question

This is not a duplicate of Why are there two articles in front of a noun?, because this time both articles are not pronouns, but real articles.

Nachdem die Schutzleute eine Weile überlegt hatten, gingen sie runter und holten eine Leiter, die sie an dem einen Hausgiebel aufstellte (…) und unter Geschrei und Gelächter lief sie (Pippi) den Dachfirst entlang zum anderen Giebel hin. ^source

A German friend of mine said that, yes, they are really articles, and the second one only shows that that Giebel (facade) is only one. But later in the text, another one appears, so that explanation doesn’t work. (The friend is a programmer, not a linguist.)

• Why are there two articles?

• Why are they in different cases?

• If the construction is correct, what is the rule, how should those be constructed and when should we use them?

A reference to some grammar source would be highly appreciated.

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Maybe, it is some intentional rule breaking for to express an emotion?

Or, maybe it is a part of the modern language (the source is the year 2004) not fitting to classical rules? For example, in Czech it easily could happen.

Answer 1

Einen in the example sentence is not an article, it is part of an idiomatic expression.

The respective part of the sentence can be translated to:

that they placed at that specific gable

In German you could also say

die sie an dem speziellen Hausgiebel aufstellten

To reply to your comment: The sentence implies that there is more than one gable. They put the letter to that one instead of the other one. The sentence could (fictionally) be continued:

die sie an dem einen Hausgiebel aufstellten, weil aus dem anderen Hausgiebel Flammen züngelten

And in the example you linked it is EXACTLY that what is said later in the text:

...die sie an dem einen Hausgiebel aufstellten... und unter Geschrei und Gelächter lief sie (Pippi) den Dachfirst entlang zum anderen Giebel hin

--> There IS another Giebel...

One more try to answer the three questions:

Q: Why are there two articles?
A: There aren't. In German there is the idiomatic expression der eine (often used together with der andere). It can be translated to the one.

Q: Why are they in different cases?
A: The rule is simple: einen takes the place of an adjective here (it describes the noun and therefore has to have this case (as described here)

Q: When to use it?
A: Whenever you use einen as adjective (meaning: this specific) and not as article.

Answer 2

This is an idiom:

Der eine (tut etwas), der andere (tut was anders).

For example, the French film "L’une chante, l’autre pas" (One sings, the other doesn't) has the German title Die eine singt, die andere nicht.

Here 'eine' is not an article; it is declined like an adjective. Hence 'dem einen' in your example. There is a strong expectation that the other gable will be mentioned shortly.