That = restrictive or defining: “The plane that was going to Chicago
has left.”
This defines which specific plane is gone.
Which = non-restrictive or non-defining: “The plane, which was going
to Chicago, has left.”
This announces the fact of the plane’s departure, and offers a bit of additional
and unnecessary information about the plane.
That, when following a noun, introduces a necessary piece of information about the
noun. Which, when following a noun, introduces information about the noun that is
not needed to understand the message.
In modern speech, many people use these words interchangeably, and are usually understood
because of tone and situational context. However, in writing, it’s essential
that we use the correct term; otherwise the meaning may be changed or missed. The
grammar checker will not catch these errors.
Some general guidelines:
• If the phrase needs a comma, you probably mean which.
• Try adding “by the way” after that or which; you wouldn’t
say, “The shoes which (by the way) I like the most are the black ones.”
The words “that” and “which” can be considered two markers
for essential and non-essential clauses. There is much debate over the usage
of these two words, as well as a good deal of crossover in usage, but for the sake
of clarity and uniformity on the site, always use “that” to signal an
essential clause and “which” to signal a non-essential clause (in grammar
books, essential and non-essential are termed restrictive and non-restrictive).
Use of that requires no comma; use of which to set off non-essential
information requires a comma before the word which.
Examples:
A pseudogene is a DNA sequence that has similar structure to an expressed
gene and is presumed to have once been functional but has acquired mutations that
render it nonfunctional.
In this case, the word that is used to restrict DNA sequence.
A pseudogene is not just a DNA sequence; a pseudogene is a DNA sequence that
has similar structure to an expressed gene. Using which in place of
that would have signaled that the clause “has similar structure to
an expressed gene . . .” was extra information and not necessary to the definition
of a pseudogene.
The most common repetitive sequence is the Alu repeat, which is 300 base
pairs in length and occurs approximately 500,000 times in the human genome.
In this case, the information that follows which is unnecessary to the understanding
of the sentence as there is only one Alu repeat, and the reader is unlikely
to need further explanation of which Alu repeat the question is referring
to. If there were more than one Alu repeat, then the phrase in the
which clause would be essential to restrict which Alu repeat was the
most common repetitive sequence – the Alu repeat that “is 300
base pairs in length. . .”
Be careful which word you choose because it is not accidental – as the writer,
you are signaling to the reader that the information that follows that or
which is essential or non-essential for meaning. As an editor, if you
have any question about whether the information is essential or non-essential, make
sure to contact the author, managing editor, or technical editor for help.
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