Many copyeditors find this confusing, especially when they are deep buried in the context but not familiar with the subject. It is difficult to decide then whether the information contained in the clause begun by a “that” or a “which” is only extra or essential.

To begin with, I will look at the basic difference. Let’s assume that I’ve gone to a nearby field with my friend. There are two cows grazing there, one brown, one red. I say to my friend:

1.a The red cow is mine.

I could have said the same thing as

1.b The cow that is red is mine.

This would simply mean that there is only one red cow out of the two (usage of the definite article) and such a cow is mine.

Now assume that there is only one cow grazing, and incidentally it is red. I now say to my friend:

1.c The cow is mine.

or

1.d The cow, which is red, is mine.

which translates to

1.e The cow you see is mine. By the way, you can also see it’s red.

“which” is for extra, parenthetical information, i.e. trivia. Note the presence of commas around “which” in this nonrestrictive usage (“nonrestrictive” since I am not restricting that only the red cow is mine; I am simply saying the cow is mine, let it be of any color, the color’s only incidental, mate!).

You will find the following:

2.a He went to Taj Mahal, which is regarded as one of the finest specimens of Mughal architecture.

You cannot say

2.b He went to Taj Mahal that is regarded as one of the finest specimens of Mughal architecture.

The whole “which … architecture” clause is in apposition to “Taj Mahal”: it describes the noun it sets off, i.e. it is just something extra being said of Taj Mahal. The main information contained is just “He went to Taj Mahal”. Of course if there would be more than one Taj Mahals, one regarded as a fine specimen and the others as bad or average specimens, then someone could have said

2.c He went to the Taj Mahal that is regarded as one of the finest specimens of Mughal architecture.

Note that the best option for an editor in cases like 2.a is the following:

2.d He went to Taj Mahal, regarded as one of the finest specimens of Mughal architecture.

This is especially true when some kind of a temporal sense need to be avoided. Refer to the following example:

3.a The scarcity of hay … led to further decrease in the already limited number of draught animals, which were essential for tilling, sowing, and transport in the absence of tractors.

I would prefer to recast the sentence as

3.b The scarcity of hay … led to further decrease in the already limited number of draught animals, essential for tilling, sowing, and transport in the absence of tractors.

Though 3.a was fine I yet choose to remove “which were”, since the necessity of draught animals for the enumerated processes would continue even today, in any society or conditions, if modern equipment (tractors here) were to be absent. Also note that some non-native speakers might write “who were essential” instead of “which were essential”: not changing “who” to “which” would be a grave omission.

Note that usage of a definite article sometimes gives hint of how essential or trivial the information is. Also, “that” is often left out in a restrictive usage, for example:

4.a Do you know the issues still left unfinished? (better)

is same as

4.b Do you know the issues that are still left unfinished?

The same also applies to “who” without a comma (restrictive) and with a comma preceding (nonrestrictive), and to “where” without a comma (restrictive) and with a comma (nonrestrictive).

5.a Taj Mahal, where hundreds of tourists go within a single day, is in Agra. (nonrestrictive; “where … day” is just extra info)

5.b He is at a place where he won’t even find a single drop of water. (restrictive; if you put comma before “where”, you are just saying “He is at a place”: that’s hardly a statement unless you were debating his existence itself)

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