Advanced Business Writing
Punctuation and Grammar

That/Which

In casual speech, the words that and which are often used interchangeably, and no one seems to mind. In your writing at work, however, it is best to use them the way they are supposed to be used.

First we consider what these words mean in terms of scope:

that = "specifically, those that"

which = "all of which"

Mr. Porter likes watches that are expensive.
[correct]

He likes a subset of all watches: the expensive ones. (Yes, we are assuming he does not like inexpensive watches; work with me here.)

Mr. Porter likes Breitling watches, which are expensive.
[correct]

There are no inexpensive Breitling watches, so using that would not make much sense. (It would be redundant.)

We can also consider what these words mean in terms of purpose:

that restricts or defines

which informs or qualifies

Let's have some fun using these words incorrectly:

Should dogs, which run free in parks, be kept on leashes?
[can be confusing]

That is equivalent to writing:

Because every dog in the world runs free in parks, we should consider keeping every dog in the world on a leash.
[not good]

The correct way to ask this question is:

Should dogs that run free in parks be kept on leashes?
[okay]

Finally, we consider what these words mean in terms of need:

that introduces a phrase that is essential to the meaning of the sentence

which introduces a phrase that is non-essential; the sentence stands alone just fine without the phrase

I only trust vegetables that are organic.
[good]

I only trust vegetables, which are organic.
[not good]

Not all vegetables are organic, so as written and punctuated, that second sentence is wrong. However, if a friend says, "I only trust vegetables which are organic," you should tell your grammar-trained mind to let it go. It's not that bad a mistake.

The use of the word which is expanding. Many writers use it in place of the word that, without the comma. Until you are the boss, you should stick with traditional usage as defined above.

Also, there are several situations where which is an acceptable alternative to that:

(1) when that has already been used in a sentence:

That is an experience which can leave you shaken.

(2) when that would otherwise be used several times in a sentence:

Ben is enrolling in a program which will qualify him for other positions and which will put him in the running for further promotions.

(3) when the essential clause is introduced by an expression such as: this X which Y, that X which Y, these X which Y, those X which Y:

We should investigate this idea which seems to have great potential.

Lesson: That/Which
Module: Punctuation and Grammar
Course: Advanced Business Writing