Why is it der Baseler?

Baseler ends in -er, and most nouns ending in -er are masculine: in a German dictionary, 86% of them take der.

All nouns ending in -er
Sense 1
der

Meaning

[1] Residents of Basel, someone who comes from Basel

[1] Bewohner von Basel, jemand der aus Basel stammt

Examples

[1] My neighbor is a born in Basele

[1] Mein Wohnungsnachbar ist ein gebürtiger Baseler.

Declension

1 Singular Plural
Nominativ der Baseler die Baseler
Genitiv des Baselers der Baseler
Dativ dem Baseler den Baselern
Akkusativ den Baseler die Baseler

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary