Q:

Comma before or after "unless"

Had a discussion with a colleague on the following sentence today:

By default situation A will happen, unless you choose to activate option B.

I have set the comma as indicated above, but my colleague is convince the comma should be after the "unless".
As a non-native speaker I don't feel I have the authority to overrule my colleague but my gut says the comma should be as indicated above. Can someone help me? Thanks.

A:

The normal usage dictates no comma at all:

By default, situation A will happen unless you choose to activate option B

(the comma after "default" is there because "by default" is an adverbial phrase, and these require a comma if they start a sentence)
Anyway, "unless" here is a dependant word, and as such does not use a comma.
However, in case of a long sentence, you may use a comma to break the sentence into more manageable chunks for easier comprehension:

By default, situation A [, including things D, E and F, as well as potential danger G,] will happen, unless you choose to activate option B [, which will disable things D and F and protect you from G].

