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As an example:

In Quantum Physics when particles are observed their behaviour changes in relation to the fact they're being watched.

Does this sentence require a comma before when and a comma before their? In other words, should it be written as:

In Quantum Physics, when particles are observed, their behaviour changes in relation to the fact they're being watched.

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The commas are not offsetting the when clause. The second comma is offsetting the when clause. The first comma is offsetting the in clause. – RegDwigнt Jun 12 '13 at 11:19
up vote 0 down vote accepted

In many respects, this is a question of style rather than of right and wrong.

Personally, I would put a comma after In Quantum Physics: that clause is not critical to the meaning of the overall sentence; and I find it helps to pause there in order to understand the sentence quickly, i.e. on the first pass.

I have no strong opinion on the second comma: on the one hand, the when clause is critical to the sentence and therefore should not be offset; on the other hand, it does make it immediately clear that the verb observed belongs to the particles and not to their behaviour.

As an aside, why does Quantum Physics justify capital letters? It is not a proper noun, merely a branch of science like many hundreds of others.

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I agree. Sometimes putting a comma makes the sentence seem more well structured, and sometimes it just interrupts the flow. – Kaiser Octavius Jun 12 '13 at 12:57

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