Is the ending -iam or -ium?

Unstressed English vowels all collapse towards the same faint sound, so the final syllable gives the ear nothing to work with: -um, -am and -om are indistinguishable at speaking speed. Latin has no such collapse. In Latin the vowel is a full u and stays one, which is why every genus with this ending is written -ium and never -iam.

An unstressed vowel, flattened until it gives the ear no information.