German has very few native suffixes that reliably signal gender. -tum is one of them. The pattern is neuter. The exceptions are two words.
The rule
Almost every noun built with the suffix -tum is neuter. The article is das. It covers abstract nouns, collective concepts, and names for states or conditions. The suffix does the work: you see it, you know the gender.
Seven words that follow the pattern:
| Word | Meaning |
|---|---|
| dasWachstum | growth |
| dasEigentum | property / ownership |
| dasAltertum | antiquity |
| dasChristentum | Christianity |
| dasJudentum | Judaism |
| dasBürgertum | bourgeoisie / citizenry |
| dasHerzogtum | duchy |
The two exceptions
Two common words break the pattern: derReichtum (wealth, riches) and derIrrtum (error, mistake). Both are masculine, and both come up regularly. Learn them as a pair.
Everything else ending in -tum is das.
A note on -um words
Latin loanwords ending in -um follow the same neuter pattern by a different route. dasZentrum (center), dasDatum (date), dasMuseum (museum), dasForum (forum) → all neuter, all das. These words kept the Latin neuter gender when they entered German. -tum and Latin -um are unrelated endings. They just happen to land on the same article.
Why it works
-tum is a German derivational suffix that creates abstract or collective nouns from other words: eigen (own) becomes Eigentum, wachsen (to grow) becomes Wachstum. Like -heit and -keit, the suffix carries gender with it. Here the gender is neuter.
Ends in -tum, the article is das. The only two exceptions are derReichtum and derIrrtum. Know those two, and you never have to guess again.