Why does “Bayreuther Festspielhaus” not inflect according to gender?

Question

Why is it that, when a city is used as an adjective, the adjective ending is (in two out of three cases) not what I expect based on the gender of the succeeding noun? Some examples:

• Bayreuther Festspielhaus, not Bayreuthes (since Haus is neuter)
• Berliner Zeitung, not Berline (since Zeitung is feminine)
• Münchner Stadtbibliothek, not Münchne (all of these "correct" versions do sound awful, I admit!)

Perhaps this isn’t just limited to city names, but they’re the examples I’ve seen. Someone once told me “because it’s genitive”, but I can’t quite follow the logic there.

In short: why are these adjective endings always -r?

And I apologize if this is a duplicate; I’m new to this SE, and I searched for this question, but perhaps the answer is hidden in one of the other (countless!) adjective ending questions.

Answer 1

The forms ending in -er that are generated from geographical names are not really adjectives. Most importantly, they are invariable, i.e. do not inflect according to case, number or presence of an article. The ending will always remain -er. Because they do not really behave as adjectives, they are capitalised unlike other adjectives.

If you want true adjectives to go with something, the ending of choice is -isch.

• Er spielt im Bayreuther Festspielhaus.

  But:

  Ich kenne ein bayreuthisches Festspielhaus.

• Es steht in der Berliner Zeitung.

  But:

  Ich lese eine berlinerische Zeitung.

• Das ist die Münchner Stadtbibliothek.

  But:

  Das ist auch eine münchnerische Stadtbibliothek.

This is not restricted to city names but can go with probably any geographic name: Schweizer Käse, Badener Hof.