Rule for feminine · die
-ur → die · 0.8% (exceptions: Abitur, Merkur, Flur)
Many nouns ending in -ur, especially those borrowed from Latin or French, are feminine and take die: die Natur (nature), die Infrastruktur (infrastructure), die Kultur (culture). The rule covers 0.8% of frequent nouns, but this one calls for real caution, because the exceptions aren't rare: das Abitur (the German school-leaving exam) and das Merkur (the planet Mercury) are neuter, while der Flur (hallway) is even masculine. With three exceptions out of 21 words ending this way, -ur is a useful rule, but not one to apply blindly - double-check unfamiliar words.