Using 'der' / 'den' without noun? Connecting sentences..
Entlang von Baden-Württembergs Westgrenze fließt der Rhein, (der / den / die) 1.320 Kilometer lang ist.

Which one is it, and why? I have looked it up in several grammar books and cannot find a solution? Also, so I can practice further, what is this situation referred to in grammar.

Thank you!!

Question

Using 'der' / 'den' without noun? Connecting sentences..
Entlang von Baden-Württembergs Westgrenze fließt der Rhein, (der / den / die) 1.320 Kilometer lang ist.

Which one is it, and why? I have looked it up in several grammar books and cannot find a solution? Also, so I can practice further, what is this situation referred to in grammar.

Thank you!!

Answer

It's "der". There's no noun following because it's not an article, but a relative pronoun: "..., *which* is 1320 km long".

http://www.canoo.net/services/OnlineGrammar/Satz/Komplex/Funktion/Attribut/Relativ/Einleitung.html
https://deutsch.lingolia.com/de/grammatik/pronomen/relativpronomen
http://www.deutschplus.net/pages/Relativpronomen_der_die_das