Should you use a comma before "whereas" and "although"?
The words "whereas" and "although" are subordinating conjunctions.
The general rule for subordinating conjunctions states that you shouldn't use a comma before a subordinating conjunction that comes after the main clause. However, "whereas" and "although" are examples of adverbs of concession, along with "though" and "even though". They are used where a dependent clause is contrasting to the main clause (a bit like "but").
You should use a comma to introduce a dependent clause that starts with an adverb of concession.
Correct: I waited at the door, whereas Katy waited in the street.
Incorrect: I waited at the door whereas Katy waited in the street.