Rule for neuter · das
-ma → das · 0.3% (but Mama, Oma, Firma are die!)
Many nouns of Greek origin ending in -ma are neuter in German: das Thema, das Klima, das Drama - and in the plural several take special forms like Themen or Dramen. Overall the pattern is fairly rare (only 0.3% of our list, 6 words), but useful for abstract or scientific terms. Here the exceptions really matter: die Firma, die Mama, and die Oma also end in -ma but are feminine - in fact among the most commonly used words in the whole vocabulary, so don't let the final letters fool you for family or business words.