Why is it der Reihen?

Reihen looks like the verb reihen used as a noun, which would be neuter. Here, though, it’s a separate word, and it takes der: der Reihen.

More verbs used as nouns
Sense 1
der

Meaning

[1] Music: outdated form for rounds, i.e. a dance song

[1] Musik: veraltete Form für Reigen, also ein Tanzlied

[2] A round dance

[2] ein Rundtanz

Examples

[1]

[1]

Declension

1 Singular Plural
Nominativ der Reihen die Reihen
Genitiv des Reihens der Reihen
Dativ dem Reihen den Reihen
Akkusativ den Reihen die Reihen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary