Editing Guide to technical writing (section)

===Which or that?===
When you are speaking "which" and "that" are interchangeable.  This is not true for writing. The rules for using "which" and "that" in writing are as follows:
* Never insert a comma before the word "that."  If you need a comma then remove "that" and replace it with "which."
* When the part of a sentence after "which" is explaining something about the sentence before the "which" then there is a comma before the "which." All other uses of "which" don't have a comma before them.

If you ever forget whether it is "that" or "which" that should not have a comma before it, a memory aid is to notice "which" has a "c" in it.
"That" has no "c" in it and think of "c" as standing for "comma."   So there is no "c" in "that" therefore no comma.   Take these two examples:

* The transistor has a negatively doped source and drain, which ensures electrons flow in the channel.
* The transistor has a negatively doped source and drain that ensures electrons flow in the channel.
* The transistor has a negatively doped source and drain to which electrons flow.

In the first example, the "which" has a comma before it, because the words after the "which" are explanatory. In the second example, the "that" obeys the no-comma rule. In the third example, "which" is making a statement not an explanation and there is no comma. Another way to think of the third example is the phrase "to which" is self-contained and can't be split by a comma.

In the following example, the sentence structure forces use of commas so we can't use "that" and we are forced to use "which" by default:

* The ultrafast microprocessor chip, which has a large power consumption, needs an adequate power supply.