15. That and Which
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.
Back to Contents
or Next Page