Which always modifies the noun or noun phrase next to it. A "noun phrase" can be a noun with a preposition modifier, such as a "group of students" or a "chain of restaurants." Usually, the verb tense of the noun by the which indicates what the "which" refers to.

So:

The tides of the Pacific Ocean, which are as predictable as their Atlantic counterparts, aided the shipping fleet.
The tides of the Pacific Ocean, which is the world's largest and deepest sea, aided the shipping fleet.

are both correct, because the plurality of the verb indicates whether the "which" clause is modifying the noun immediately preceding the comma, or the entire noun phrase.

Incorrect examples will either separate the "which" modifier from the proper noun with a verb, or use a verb tense that doesn't clearly refer to one or the other. The following are wrong:

The tides of the Pacific and Atlantic Oceans, which are large and powerful, aid shipping fleets around the world.
The tides of the Pacific Ocean aided the shipping fleet, which are highly predictable.

Hope this helps!

Hi Eli Meyer,

Thanks for a major clarification

Few more questions:

1) In GMAT is it safe to consider that always "which" should a "," before it
2)I have seen few questions in OG where correct answers had "in which" in them.

For Ex In OG:

His studies of ice polished rocks in his alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets existed in what are now temperate areas.

Kindly clarify regarding usage of "in which"

Thanks

venjam wrote:Hi Eli Meyer,

Thanks for a major clarification

Few more questions:

1) In GMAT is it safe to consider that always "which" should a "," before it
2)I have seen few questions in OG where correct answers had "in which" in them.

For Ex In OG:

His studies of ice polished rocks in his alpine homeland, far outside the range of present-day glaciers, led Louis Agassiz in 1837 to propose the concept of an age in which great ice sheets existed in what are now temperate areas.

Kindly clarify regarding usage of "in which"

Thanks

When "which" is on it's own, it will always be an appositive phrase--one set off by commas on both sides. However, the Official Guide explicitly states that this will not be tested; this written convention is not universally accepted by scholars of the English language.

When "which" is accompanied by a preposition, such as "in which" or "of which," it does not need to be preceded by a comma. Generally, this form is used when we would use "Where" or "When" in spoken English. For example, "If you are in a situation where you are running out of time, take a deep breath and refocus" sounds natural when read aloud, but isn't acceptable written English. "Where" must refer only to a literal physical location. It should be "in which."