No rule for this one
No ending or word group predicts the article of Bett. Learn it as a pair and say it out loud: das Bett.
das Bett
It's das Bett. Bett is neuter, plural die Betten.
Das Bett ist bequem.
The bed is comfortable.
No ending or word group predicts the article of Bett. Learn it as a pair and say it out loud: das Bett.