- A Comma for a Clause -
The rule about a comma preceding a conjunction or an independent clause applies equally well for other conjunctions such as and, or, yet, nor, yet, for, as [because], and while [at the same time]:
Twice each year they battle the high waters, for the Amazon basin has biannual rainy seasons.
The villagers were worried, as they had never seen such a flood.
They stared at the moon's reflection in the water, while the raft drifted downstream.